The protamine family of sperm nuclear proteins

Abstract

The protamines are a diverse family of small arginine-rich proteins that are synthesized in the late-stage spermatids of many animals and plants and bind to DNA, condensing the spermatid genome into a genetically inactive state. Vertebrates have from one to 15 protamine genes per haploid genome, which are clustered together on the same chromosome. Comparison of protamine gene and amino-acid sequences suggests that the family evolved from specialized histones through protamine-like proteins to the true protamines. Structural elements present in all true protamines are a series of arginine-rich DNA-anchoring domains (often containing a mixture of arginine and lysine residues in non-mammalian protamines) and multiple phosphorylation sites. The two protamines found in mammals, P1 and P2, are the most widely studied. P1 packages sperm DNA in all mammals, whereas protamine P2 is present only in the sperm of primates, many rodents and a subset of other placental mammals. P2, but not P1, is synthesized as a precursor that undergoes proteolytic processing after binding to DNA and also binds a zinc atom, the function of which is not known. P1 and P2 are phosphorylated soon after their synthesis, but after binding to DNA most of the phosphate groups are removed and cysteine residues are oxidized, forming disulfide bridges that link the protamines together. Both P1 and P2 have been shown to be required for normal sperm function in primates and many rodents.

Figure 1

Primary structures of mouse protamine genes and proteins. Schematic representation of the mouse (a) mP1 and (b) mP2 proteins. Numbers denote amino acid residues; the two exons in each protein are shown on bars below the proteins; important residues are indicated as shown in the key (using letters in the single-letter amino acid code, with a subscript number indicating the residue number). And, proposed DNA anchoring domains (regions containing 3 or more consecutive arginine and lysine residues). Proposed phosphorylation sites are amino acid residues identified to be phosphorylated in proteins isolated from sperm or following in vitro incubation of the isolated proteins of five mammalian species with cAMP-dependent protein kinase or protein kinase C.

Figure 2

Protamine molecules bind in the major groove of DNA, neutralizing the phosphodiester backbone of DNA and causing the DNA molecules to coil into toroidal structures. (a) Model showing how two adjacent salmon protamine molecules (blue atoms) wrap around the DNA helix (white atoms) and bind within the major groove of DNA. (b) Scanning-probe images of toroidal DNA-protamine complexes prepared in vitro on a graphite surface by adding protamine to DNA attached loosely to the surface. The toroids formed in vitro are similar in size and shape to those isolated from human sperm chromatin (c). (c) Scanning-probe microscope images of native DNA-protamine toroids obtained from human sperm chromatin. These toroids, which comprise the basic subunit structure of protamine-bound DNA, contain approximately 50,000 bp of DNA coiled into each donut-shaped structure.