sturgeon (Acipenser stellatus) scylliorhinine from dogfish (Scylliorhinus) Fish protamine are generally shorter than that of mammals, with a higher amount of arginine. Sequence The primary structure of protamine P1, the protamine used for packaging DNA in sperm cells, in placental mammals is usually 49 or 50 amino acids long. This sequence is divided into three separate domains: an arginine-rich domain for DNA binding flanked by shorter peptide sequences containing mostly cysteine residues. The arginine-rich domain consists of 3-11 arginine residues and is conserved between fish protamine and mammalian protamine 1 sequences at about 60-80% sequence identity. Structure After translation, the protamine P1 structure is immediately phosphorylated at all three of the above-mentioned domains. Another round of phosphorylation occurs when the sperm enters the egg, but the function of these phosphorylations is uncertain. The exact secondary and tertiary structure of protamine is not known with certainty, but several proposals have been published since the 1970s. The broad consensus is that protamine forms beta strand structures that then crosslink through disulfide bonds (and potentially dityrosine and cysteine-tyrosine bonds). When protamine P1 binds to DNA, cysteine from the amino terminal of one protamine P1 forms disulfide bonds with the cysteine from the carboxy-terminal of another protamine P1. By neutralizing the backbone charge protamine enables the DNA to more tightly coil. The disulfide bonds function to prevent the dissociation of protamine P1 from DNA until the bonds are reduced when the sperm enters the egg. These long protamine polymers may then wrap around the DNA within the major groove. See also Epigenetics Chromosome References External links Proteins