Datasets:
Reproducibility
/
naacl22_wikitext15M

Dataset card Data Studio Files Community
text
stringlengths
0
7.06k
Several factors led to the Frank Slide . A study conducted by the GSC immediately following the slide concluded that the primary cause was the mountain 's unstable anticline formation ; a layer of limestone rested on top of softer materials that , after years of erosion , resulted in a top @-@ heavy , steep cliff . Cracks laced the eastern face of the mountain while underground fissures allowed water to flow into the mountain 's core . Local Indigenous peoples of the area , the Blackfoot and Ktunaxa , had oral traditions referring to the peak as " the mountain that moves . " Miners noticed the mountain had become increasingly unstable in the months preceding the slide ; they felt small tremors and the superintendent reported a " general squeeze " in the mountain at depths between 1 @,@ 100 metres ( 3 @,@ 600 ft ) and 1 @,@ 500 metres ( 4 @,@ 900 ft ) . They found that coal broke from its seam ; it was said to have practically mined itself .
An unusually warm winter , with warm days and cold nights , was also a factor . Water in the mountain 's fissures froze and thawed repeatedly , further weakening the mountain 's supports . Heavy snowfall in the region in March was followed by a warm April , causing the mountain snows to melt into the fissures . GSC geologists concluded that the weather conditions that night likely triggered the slide . The crew of the freight train that arrived at Frank shortly before the disaster said it was the coldest night of the winter , with overnight temperatures falling below βˆ’ 18 Β° C ( 0 Β° F ) . Geologists speculated that the cold snap and rapid freezing resulted in expansion of the fissures , causing the limestone to break off and tumble down the mountain .
Though the GSC concluded that mining activities contributed to the slide , the facility 's owners disagreed . Their engineers claimed that the mine bore no responsibility . Later studies suggested that the mountain had been at a point of " equilibrium " ; even a small deformation such as that caused by the mine 's existence would have helped trigger a slide . The mine was quickly re @-@ opened , even though rock continued to tumble down the mountain . Coal production at Frank peaked in 1910 , but the mine was permanently closed in 1917 after it became unprofitable .
The slide created two new peaks on the mountain ; the south peak stands 2 @,@ 200 metres ( 7 @,@ 200 ft ) high and the north peak 2 @,@ 100 metres ( 6 @,@ 900 ft ) . Geologists believe that another slide is inevitable , though not imminent . The south peak is considered the most likely to fall ; it would likely create a slide about one @-@ sixth the size of the 1903 slide . The mountain , continuously monitored for changes in stability , has been studied on numerous occasions . The Alberta Geological Survey operates a state @-@ of @-@ the @-@ art monitoring system used by researchers around the world . Over 80 monitoring stations have been placed on the face of the mountain to provide an early warning system for area residents in case of another slide .
Geologists have debated about what caused the slide debris to travel the distance it did . The " air cushion " theory , an early hypothesis , postulated that a layer of air was trapped between the mass of rock and the mountain , which caused the rock to move a greater distance than would otherwise be expected . " Acoustic <unk> " is another theory , which suggests that large masses of material create seismic energy that reduces friction and causes the debris to flow down the mountain as though it is a fluid . Geologists created the term " debris avalanche " to describe the Frank Slide .
= = Legends = =
Numerous legends and misconceptions were spawned in the aftermath of the slide . The entire town of Frank was claimed to have been buried , though much of the town itself was unscathed . The belief that a branch of the Union Bank of Canada had been buried with as much as $ 500 @,@ 000 persisted for many years . The bank β€” untouched by the slide β€” remained in the same location until it was demolished in 1911 , after which the buried treasure legend arose . Crews building a new road through the pass in 1924 operated under police guard as it was believed they could unearth the supposedly buried bank .
Several people , telling amazing stories to those who would listen , passed themselves off as the " sole survivor " in the years following the slide . The most common such tale is that of an infant girl said to have been the only survivor of the slide . Her real name unknown , the girl was called " Frankie Slide " . Several stories were told of her miraculous escape : she was found in a bale of hay , lying on rocks , under the collapsed roof of her house or in the arms of her dead mother . The legend was based primarily on the story of Marion Leitch , who was thrown from her home into a pile of hay when the slide enveloped her home . Her sisters also survived ; they were found unharmed under a collapsed ceiling joist . Her parents and four brothers died . Influencing the story was the survival of two @-@ year @-@ old Gladys Ennis , who was found outside her home in the mud . The last survivor of the slide , she died in 1995 . In total , 23 people in the path of the slide survived , in addition to the 17 miners who escaped from the tunnels under Turtle Mountain . A ballad by Ed McCurdy featuring the story of Frankie Slide was popular in parts of Canada in the 1950s . The slide has formed the basis of other songs , including " How the Mountain Came Down " by Stompin ' Tom Connors , and more recently , " Frank , AB " by The Rural Alberta Advantage . The Frank Slide has been the subject of several books , both historical and fictional .
= = Legacy = =
Curious sightseers flocked to the site of the slide within the day of the disaster . It has remained a popular tourist destination , in part due to its proximity to the Crowsnest Highway . The province built a roadside turnout in 1941 to accommodate the traffic . Town boosters unsuccessfully sought to have the site designated as a National Historic Site in 1958 . It was later designated a Provincial Historic Site of Alberta . The provincial government designated the slide area a restricted development zone in 1976 , which prevents alteration of the site . In 1978 , a memorial plaque was erected . The Frank Slide Interpretive Centre , within sight of the mountain , was opened in 1985 . A museum and tourist stop document the Frank Slide and the region 's coal mining history . The site receives over 100 @,@ 000 tourist visits annually .
Though Frank recovered from the slide and achieved a peak population of 1 @,@ 000 shortly thereafter , the closure of the mine resulted in a longstanding decline in population . Frank ceased to be an independent community in 1979 when it was amalgamated into the Municipality of Crowsnest Pass along with the neighbouring communities of Blairmore , Coleman , Hillcrest and Bellevue . Frank is now home to about 200 residents .
= Protein =
Proteins ( / <unk> / or / <unk> / ) are large biomolecules , or macromolecules , consisting of one or more long chains of amino acid residues . Proteins perform a vast array of functions within organisms , including catalysing metabolic reactions , DNA replication , responding to stimuli , and transporting molecules from one location to another . Proteins differ from one another primarily in their sequence of amino acids , which is dictated by the nucleotide sequence of their genes , and which usually results in protein folding into a specific three @-@ dimensional structure that determines its activity .
A linear chain of amino acid residues is called a polypeptide . A protein contains at least one long polypeptide . Short polypeptides , containing less than 20 @-@ 30 residues , are rarely considered to be proteins and are commonly called peptides , or sometimes <unk> . The individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues . The sequence of amino acid residues in a protein is defined by the sequence of a gene , which is encoded in the genetic code . In general , the genetic code specifies 20 standard amino acids ; however , in certain organisms the genetic code can include selenocysteine and β€” in certain archaea β€” pyrrolysine . Shortly after or even during synthesis , the residues in a protein are often chemically modified by post @-@ translational modification , which alters the physical and chemical properties , folding , stability , activity , and ultimately , the function of the proteins . Sometimes proteins have non @-@ peptide groups attached , which can be called prosthetic groups or cofactors . Proteins can also work together to achieve a particular function , and they often associate to form stable protein complexes .
Once formed , proteins only exist for a certain period of time and are then degraded and recycled by the cell 's machinery through the process of protein turnover . A protein 's lifespan is measured in terms of its half @-@ life and covers a wide range . They can exist for minutes or years with an average lifespan of 1 – 2 days in mammalian cells . Abnormal and or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable .
Like other biological macromolecules such as polysaccharides and nucleic acids , proteins are essential parts of organisms and participate in virtually every process within cells . Many proteins are enzymes that catalyse biochemical reactions and are vital to metabolism . Proteins also have structural or mechanical functions , such as actin and myosin in muscle and the proteins in the cytoskeleton , which form a system of scaffolding that maintains cell shape . Other proteins are important in cell signaling , immune responses , cell adhesion , and the cell cycle . In animals , proteins are needed in the diet to provide the essential amino acids that cannot be synthesized . Digestion breaks the proteins down for use in the metabolism .
Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation , precipitation , electrophoresis , and chromatography ; the advent of genetic engineering has made possible a number of methods to facilitate purification . Methods commonly used to study protein structure and function include immunohistochemistry , site @-@ directed mutagenesis , X @-@ ray crystallography , nuclear magnetic resonance and mass spectrometry .
= = Biochemistry = =
Most proteins consist of linear polymers built from series of up to 20 different L @-@ Ξ± @-@ amino acids . All proteinogenic amino acids possess common structural features , including an Ξ± @-@ carbon to which an amino group , a carboxyl group , and a variable side chain are bonded . Only proline differs from this basic structure as it contains an unusual ring to the N @-@ end amine group , which forces the CO – NH amide moiety into a fixed conformation . The side chains of the standard amino acids , detailed in the list of standard amino acids , have a great variety of chemical structures and properties ; it is the combined effect of all of the amino acid side chains in a protein that ultimately determines its three @-@ dimensional structure and its chemical reactivity . The amino acids in a polypeptide chain are linked by peptide bonds . Once linked in the protein chain , an individual amino acid is called a residue , and the linked series of carbon , nitrogen , and oxygen atoms are known as the main chain or protein backbone .
The peptide bond has two resonance forms that contribute some double @-@ bond character and inhibit rotation around its axis , so that the alpha carbons are roughly coplanar . The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone . The end of the protein with a free carboxyl group is known as the C @-@ terminus or carboxy terminus , whereas the end with a free amino group is known as the N @-@ terminus or amino terminus . The words protein , polypeptide , and peptide are a little ambiguous and can overlap in meaning . Protein is generally used to refer to the complete biological molecule in a stable conformation , whereas peptide is generally reserved for a short amino acid oligomers often lacking a stable three @-@ dimensional structure . However , the boundary between the two is not well defined and usually lies near 20 – 30 residues . <unk> can refer to any single linear chain of amino acids , usually regardless of length , but often implies an absence of a defined conformation .
= = = Abundance in cells = = =
It has been estimated that average @-@ sized bacteria contain about 2 million proteins per cell ( e.g. E. coli and Staphylococcus aureus ) . Smaller bacteria , such as Mycoplasma or spirochetes contain fewer molecules , namely on the order of 50 @,@ 000 to 1 million . By contrast , eukaryotic cells are larger and thus contain much more protein . For instance , yeast cells were estimated to contain about 50 million proteins and human cells on the order of 1 to 3 billion . Note that bacterial genomes encode about 10 times fewer proteins than humans ( e.g. small bacteria ~ 1 @,@ 000 , E. coli : ~ 4 @,@ 000 , yeast : ~ 6 @,@ 000 , human : ~ 20 @,@ 000 ) .
Importantly , the concentration of individual proteins ranges from a few molecules per cell to hundreds of thousands . In fact , about a third of all proteins is not produced in most cells or only induced under certain circumstances . For instance , of the 20 @,@ 000 or so proteins encoded by the human genome only 6 @,@ 000 are detected in <unk> cells .
= = Synthesis = =
= = = Biosynthesis = = =
Proteins are assembled from amino acids using information encoded in genes . Each protein has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene encoding this protein . The genetic code is a set of three @-@ nucleotide sets called codons and each three @-@ nucleotide combination designates an amino acid , for example AUG ( adenine @-@ uracil @-@ guanine ) is the code for methionine . Because DNA contains four nucleotides , the total number of possible codons is 64 ; hence , there is some redundancy in the genetic code , with some amino acids specified by more than one codon . Genes encoded in DNA are first transcribed into pre @-@ messenger RNA ( mRNA ) by proteins such as RNA polymerase . Most organisms then process the pre @-@ mRNA ( also known as a primary transcript ) using various forms of Post @-@ transcriptional modification to form the mature mRNA , which is then used as a template for protein synthesis by the ribosome . In prokaryotes the mRNA may either be used as soon as it is produced , or be bound by a ribosome after having moved away from the nucleoid . In contrast , eukaryotes make mRNA in the cell nucleus and then translocate it across the nuclear membrane into the cytoplasm , where protein synthesis then takes place . The rate of protein synthesis is higher in prokaryotes than eukaryotes and can reach up to 20 amino acids per second .
The process of synthesizing a protein from an mRNA template is known as translation . The mRNA is loaded onto the ribosome and is read three nucleotides at a time by matching each codon to its base pairing anticodon located on a transfer RNA molecule , which carries the amino acid corresponding to the codon it recognizes . The enzyme aminoacyl tRNA synthetase " charges " the tRNA molecules with the correct amino acids . The growing polypeptide is often termed the nascent chain . Proteins are always biosynthesized from N @-@ terminus to C @-@ terminus .
The size of a synthesized protein can be measured by the number of amino acids it contains and by its total molecular mass , which is normally reported in units of daltons ( synonymous with atomic mass units ) , or the derivative unit <unk> ( kDa ) . Yeast proteins are on average 466 amino acids long and 53 kDa in mass . The largest known proteins are the <unk> , a component of the muscle <unk> , with a molecular mass of almost 3 @,@ 000 kDa and a total length of almost 27 @,@ 000 amino acids .
= = = Chemical synthesis = = =
Short proteins can also be synthesized chemically by a family of methods known as peptide synthesis , which rely on organic synthesis techniques such as chemical ligation to produce peptides in high yield . Chemical synthesis allows for the introduction of non @-@ natural amino acids into polypeptide chains , such as attachment of fluorescent probes to amino acid side chains . These methods are useful in laboratory biochemistry and cell biology , though generally not for commercial applications . Chemical synthesis is inefficient for polypeptides longer than about 300 amino acids , and the synthesized proteins may not readily assume their native tertiary structure . Most chemical synthesis methods proceed from C @-@ terminus to N @-@ terminus , opposite the biological reaction .
= = Structure = =
Most proteins fold into unique 3 @-@ dimensional structures . The shape into which a protein naturally folds is known as its native conformation . Although many proteins can fold unassisted , simply through the chemical properties of their amino acids , others require the aid of molecular chaperones to fold into their native states . <unk> often refer to four distinct aspects of a protein 's structure :
Primary structure : the amino acid sequence . A protein is a <unk> .
Secondary structure : regularly repeating local structures stabilized by hydrogen bonds . The most common examples are the Ξ± @-@ helix , Ξ² @-@ sheet and turns . Because secondary structures are local , many regions of different secondary structure can be present in the same protein molecule .
Tertiary structure : the overall shape of a single protein molecule ; the spatial relationship of the secondary structures to one another . Tertiary structure is generally stabilized by nonlocal interactions , most commonly the formation of a hydrophobic core , but also through salt bridges , hydrogen bonds , disulfide bonds , and even posttranslational modifications . The term " tertiary structure " is often used as synonymous with the term fold . The tertiary structure is what controls the basic function of the protein .
Quaternary structure : the structure formed by several protein molecules ( polypeptide chains ) , usually called protein subunits in this context , which function as a single protein complex .
Proteins are not entirely rigid molecules . In addition to these levels of structure , proteins may shift between several related structures while they perform their functions . In the context of these functional rearrangements , these tertiary or quaternary structures are usually referred to as " conformations " , and transitions between them are called conformational changes . Such changes are often induced by the binding of a substrate molecule to an enzyme 's active site , or the physical region of the protein that participates in chemical catalysis . In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules .
Proteins can be informally divided into three main classes , which correlate with typical tertiary structures : globular proteins , fibrous proteins , and membrane proteins . Almost all globular proteins are soluble and many are enzymes . <unk> proteins are often structural , such as collagen , the major component of connective tissue , or keratin , the protein component of hair and nails . Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane .
A special case of intramolecular hydrogen bonds within proteins , poorly shielded from water attack and hence promoting their own dehydration , are called <unk> .
= = = Structure determination = = =
Discovering the tertiary structure of a protein , or the quaternary structure of its complexes , can provide important clues about how the protein performs its function . Common experimental methods of structure determination include X @-@ ray crystallography and NMR spectroscopy , both of which can produce information at atomic resolution . However , NMR experiments are able to provide information from which a subset of distances between pairs of atoms can be estimated , and the final possible conformations for a protein are determined by solving a distance geometry problem . Dual polarisation interferometry is a quantitative analytical method for measuring the overall protein conformation and conformational changes due to interactions or other stimulus . Circular dichroism is another laboratory technique for determining internal Ξ² @-@ sheet / Ξ± @-@ helical composition of proteins . <unk> microscopy is used to produce lower @-@ resolution structural information about very large protein complexes , including assembled viruses ; a variant known as electron crystallography can also produce high @-@ resolution information in some cases , especially for two @-@ dimensional crystals of membrane proteins . Solved structures are usually deposited in the Protein Data Bank ( PDB ) , a freely available resource from which structural data about thousands of proteins can be obtained in the form of Cartesian coordinates for each atom in the protein .
Many more gene sequences are known than protein structures . Further , the set of solved structures is biased toward proteins that can be easily subjected to the conditions required in X @-@ ray crystallography , one of the major structure determination methods . In particular , globular proteins are comparatively easy to crystallize in preparation for X @-@ ray crystallography . Membrane proteins , by contrast , are difficult to crystallize and are underrepresented in the PDB . Structural genomics initiatives have attempted to remedy these deficiencies by systematically solving representative structures of major fold classes . Protein structure prediction methods attempt to provide a means of generating a plausible structure for proteins whose structures have not been experimentally determined .
= = Cellular functions = =
Proteins are the chief actors within the cell , said to be carrying out the duties specified by the information encoded in genes . With the exception of certain types of RNA , most other biological molecules are relatively inert elements upon which proteins act . Proteins make up half the dry weight of an Escherichia coli cell , whereas other macromolecules such as DNA and RNA make up only 3 % and 20 % , respectively . The set of proteins expressed in a particular cell or cell type is known as its proteome .
The chief characteristic of proteins that also allows their diverse set of functions is their ability to bind other molecules specifically and tightly . The region of the protein responsible for binding another molecule is known as the binding site and is often a depression or " pocket " on the molecular surface . This binding ability is mediated by the tertiary structure of the protein , which defines the binding site pocket , and by the chemical properties of the surrounding amino acids ' side chains . Protein binding can be extraordinarily tight and specific ; for example , the ribonuclease inhibitor protein binds to human <unk> with a sub @-@ <unk> dissociation constant ( < 10 βˆ’ 15 M ) but does not bind at all to its amphibian homolog <unk> ( > 1 M ) . Extremely minor chemical changes such as the addition of a single methyl group to a binding partner can sometimes suffice to nearly eliminate binding ; for example , the aminoacyl tRNA synthetase specific to the amino acid valine discriminates against the very similar side chain of the amino acid isoleucine .
Proteins can bind to other proteins as well as to small @-@ molecule substrates . When proteins bind specifically to other copies of the same molecule , they can <unk> to form fibrils ; this process occurs often in structural proteins that consist of globular monomers that self @-@ associate to form rigid fibers . Protein – protein interactions also regulate enzymatic activity , control progression through the cell cycle , and allow the assembly of large protein complexes that carry out many closely related reactions with a common biological function . Proteins can also bind to , or even be integrated into , cell membranes . The ability of binding partners to induce conformational changes in proteins allows the construction of enormously complex signaling networks . Importantly , as interactions between proteins are reversible , and depend heavily on the availability of different groups of partner proteins to form aggregates that are capable to carry out discrete sets of function , study of the interactions between specific proteins is a key to understand important aspects of cellular function , and ultimately the properties that distinguish particular cell types .
= = = Enzymes = = =
The best @-@ known role of proteins in the cell is as enzymes , which catalyse chemical reactions . Enzymes are usually highly specific and accelerate only one or a few chemical reactions . Enzymes carry out most of the reactions involved in metabolism , as well as manipulating DNA in processes such as DNA replication , DNA repair , and transcription . Some enzymes act on other proteins to add or remove chemical groups in a process known as posttranslational modification . About 4 @,@ 000 reactions are known to be catalysed by enzymes . The rate acceleration conferred by enzymatic catalysis is often enormous β€” as much as 1017 @-@ fold increase in rate over the uncatalysed reaction in the case of orotate decarboxylase ( 78 million years without the enzyme , 18 milliseconds with the enzyme ) .
The molecules bound and acted upon by enzymes are called substrates . Although enzymes can consist of hundreds of amino acids , it is usually only a small fraction of the residues that come in contact with the substrate , and an even smaller fraction β€” three to four residues on average β€” that are directly involved in catalysis . The region of the enzyme that binds the substrate and contains the catalytic residues is known as the active site .
<unk> proteins are members of a class of proteins that dictate the stereochemistry of a compound synthesized by other enzymes .
= = = Cell signaling and ligand binding = = =
Many proteins are involved in the process of cell signaling and signal transduction . Some proteins , such as insulin , are extracellular proteins that transmit a signal from the cell in which they were synthesized to other cells in distant tissues . Others are membrane proteins that act as receptors whose main function is to bind a signaling molecule and induce a biochemical response in the cell . Many receptors have a binding site exposed on the cell surface and an effector domain within the cell , which may have enzymatic activity or may undergo a conformational change detected by other proteins within the cell .
Antibodies are protein components of an adaptive immune system whose main function is to bind antigens , or foreign substances in the body , and target them for destruction . Antibodies can be secreted into the extracellular environment or anchored in the membranes of specialized B cells known as plasma cells . Whereas enzymes are limited in their binding affinity for their substrates by the necessity of conducting their reaction , antibodies have no such constraints . An antibody 's binding affinity to its target is extraordinarily high .
Many ligand transport proteins bind particular small biomolecules and transport them to other locations in the body of a multicellular organism . These proteins must have a high binding affinity when their ligand is present in high concentrations , but must also release the ligand when it is present at low concentrations in the target tissues . The canonical example of a ligand @-@ binding protein is haemoglobin , which transports oxygen from the lungs to other organs and tissues in all vertebrates and has close homologs in every biological kingdom . Lectins are sugar @-@ binding proteins which are highly specific for their sugar moieties . Lectins typically play a role in biological recognition phenomena involving cells and proteins . Receptors and hormones are highly specific binding proteins .
<unk> proteins can also serve as ligand transport proteins that alter the permeability of the cell membrane to small molecules and ions . The membrane alone has a hydrophobic core through which polar or charged molecules cannot diffuse . Membrane proteins contain internal channels that allow such molecules to enter and exit the cell . Many ion channel proteins are specialized to select for only a particular ion ; for example , potassium and sodium channels often discriminate for only one of the two ions .
= = = Structural proteins = = =
Structural proteins confer stiffness and rigidity to otherwise @-@ fluid biological components . Most structural proteins are fibrous proteins ; for example , collagen and elastin are critical components of connective tissue such as cartilage , and keratin is found in hard or filamentous structures such as hair , nails , feathers , hooves , and some animal shells . Some globular proteins can also play structural functions , for example , actin and tubulin are globular and soluble as monomers , but polymerize to form long , stiff fibers that make up the cytoskeleton , which allows the cell to maintain its shape and size .
Other proteins that serve structural functions are motor proteins such as myosin , kinesin , and dynein , which are capable of generating mechanical forces . These proteins are crucial for cellular motility of single celled organisms and the sperm of many multicellular organisms which reproduce sexually . They also generate the forces exerted by contracting muscles and play essential roles in intracellular transport .
= = Methods of study = =
The activities and structures of proteins may be examined in vitro , in vivo , and in silico . In vitro studies of purified proteins in controlled environments are useful for learning how a protein carries out its function : for example , enzyme kinetics studies explore the chemical mechanism of an enzyme 's catalytic activity and its relative affinity for various possible substrate molecules . By contrast , in vivo experiments can provide information about the physiological role of a protein in the context of a cell or even a whole organism . In silico studies use computational methods to study proteins .
= = = Protein purification = = =
To perform in vitro analysis , a protein must be purified away from other cellular components . This process usually begins with cell lysis , in which a cell 's membrane is disrupted and its internal contents released into a solution known as a crude lysate . The resulting mixture can be purified using ultracentrifugation , which <unk> the various cellular components into fractions containing soluble proteins ; membrane lipids and proteins ; cellular organelles , and nucleic acids . Precipitation by a method known as salting out can concentrate the proteins from this lysate . Various types of chromatography are then used to isolate the protein or proteins of interest based on properties such as molecular weight , net charge and binding affinity . The level of purification can be monitored using various types of gel electrophoresis if the desired protein 's molecular weight and isoelectric point are known , by spectroscopy if the protein has distinguishable spectroscopic features , or by enzyme assays if the protein has enzymatic activity . Additionally , proteins can be isolated according their charge using <unk> .
For natural proteins , a series of purification steps may be necessary to obtain protein sufficiently pure for laboratory applications . To simplify this process , genetic engineering is often used to add chemical features to proteins that make them easier to purify without affecting their structure or activity . Here , a " tag " consisting of a specific amino acid sequence , often a series of histidine residues ( a " His @-@ tag " ) , is attached to one terminus of the protein . As a result , when the lysate is passed over a chromatography column containing nickel , the histidine residues <unk> the nickel and attach to the column while the untagged components of the lysate pass unimpeded . A number of different tags have been developed to help researchers purify specific proteins from complex mixtures .
= = = Cellular localization = = =