apical loop of helix 8 contains four, five, or six residues, depending on the species. It has a highly conserved guanosine as the first and an adenosine as the last loop residue. This feature is required for the interaction with the third adenosine residue of the helix 6 GNAR tetraloop motif. Helix 6 GNAR tetraloop The SRP RNAs of eukaryotes and Archaea have a GNAR tetraloop (N is for any nucleotide, R is for a purine) in helix 6. Its conserved adenosine residue is important for the binding of protein SRP19. This adenosine makes a tertiary interaction with another adenosine residue located in the apical loop of helix 8. 5e The 11 nucleotides of the 5e motif form four base pairs which are interrupted by a loop of three nucleotides. In the eukaryotes, the first nucleotide of the loop is an adenosine which is needed for the binding of protein SRP72. UGU(NR) The UGU(NR) motif connects helices 3 and 4 in the small (Alu) SRP domain. Fungal SRP RNAs lacking helices 3 and 4 contain the motif within the loop of helix 2. It is important in the binding of the SRP9/14 protein heterodimer as part of an RNA U-turn. Secondary Tertiary X-ray crystallography, nuclear magnetic resonance (NMR), and cryo-electron microscopy (cryo-EM] have been used to determine the molecular structure of portions of the SRP RNAs from various species. The available PDB structures show the RNA molecule either free or when bound to one or more SRP proteins. Binding proteins One or more SRP proteins bind to the SRP RNA to assemble the functional SRP. The SRP proteins are named according to their approximate molecular mass measured in kilodalton. Most bacterial SRPs are composed of SRP RNA and SRP54 (also named Ffh for "Fifty-four homolog"). The Archaeal SRP contains proteins SRP54 and SRP19. In eukaryotes, the SRP RNA combines with the imported SRP proteins SRP9/14, SRP19, and SRP68/72 in a region of the nucleolus. This pre-SRP is transported to the cytosol where it binds to protein SRP54. The molecular structures of the free or SRP RNA-bound proteins SRP9/14, SRP19, or SRP54 are known at high resolution. SRP9 and SRP14 SRP9 and SRP14 are structurally related and form the SRP9/14 heterodimer which binds to the SRP RNA of the small (Alu) domain. Yeast SRP lacks SRP9 and contains the structurally related binding protein SRP21. Yeast SRP14 forms homodimers in crystal and