protaminas

Protaminas, or protamines, are small, highly basic nuclear proteins that package the DNA of spermatozoa in many animals, most prominently in mammals. They replace histones during spermiogenesis and enable extreme condensation of the paternal genome, producing a compact chromatin that protects DNA during transit and fertilization.

Biochemical properties: Protamines are rich in arginine and other positively charged residues; they are synthesized as larger precursor proteins and processed to mature forms. In humans, the two main protamines are protamine 1 and protamine 2. They also form disulfide crosslinks via cysteine residues, which stabilizes the highly condensed chromatin.

Genetics and expression: In humans, the genes PRM1 and PRM2 encode the two protamines and are expressed specifically in testis. They are transcribed late in spermatogenesis, translated as precursors, and processed into mature protamines that are loaded onto sperm DNA in the final stages of chromatin condensation.

Function and fertilization: After sperm maturation, protamines replace histones and transition proteins to achieve tight packaging of the haploid genome. After fertilization, protamine-containing paternal chromatin undergoes remodeling to reestablish a histone-based chromatin state in the embryo, contributing to epigenetic inheritance.

Clinical and evolutionary notes: Abnormal protamine ratios or expression are associated with male infertility and poor embryo quality in humans; protamine genes evolve rapidly and vary between species, reflecting diverse reproductive strategies. Protamine studies inform fertility diagnostics and contribute to understanding sperm chromatin biology.