Protamines are small arginine-rich proteins that condense DNA in sperm to almost crystalline packing levels [1], creating a more hydrodynamic sperm head for efficient swimming and protects the DNA from UV radiation and DNA damage [1,2]. These proteins bind non-specifically to the DNA and use a multi-step process to loop the DNA [1]. During sperm development, the histones that package DNA in early spermatids are removed from the DNA and replaced in the final stages of spermatid maturation by sperm-specific histones, protamine-like proteins or protamines [2]. Sequence comparisons of vertebrate and invertebrate protamines show that the sequence, structure, and probably function of these proteins evolve independently in vertebrates and various invertebrate groups [2].

1: Protamine loops DNA in multiple steps. Ukogu OA, Smith AD, Devenica LM, Bediako H, McMillan RB, Ma Y, Balaji A, Schwab RD, Anwar S, Dasgupta M, Carter AR; Nucleic Acids Res. 2020;48:6108-6119. PMID:32392345

2: The protamine family of sperm nuclear proteins. Balhorn R; Genome Biol. 2007;8:227. PMID:17903313