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protamine
Protamines are small, arginine-rich, nuclear proteins that replace histones late in the haploid phase of spermatogenesis and are believed essential for sperm head condensation and DNA stabilization. They may allow for denser packaging of DNA in spermatozoon than histones, but they must be decompressed before the genetic data can be used for protein synthesis. However, in humans and maybe other primates, 10-15% of the sperm's genome is packaged by histones thought to bind genes that are essential for early embryonic development.
== Spermatogenesis ==
During the formation of sperm, protamine binds to the phosphate backbone of DNA using the arginine-rich domain as an anchor. DNA is then folded into a toroid, an O-shaped structure, although the mechanism is not known. A sperm cell can contain up to 50,000 toroid-shaped structures in its nucleus with each toroid containing about 50 kilobases. Before the toroid is formed, histones are removed from the DNA by transition nuclear proteins, so that protamine can condense it. The effects of this change 1) increase in sperm hydrodynamics for better flow through liquids by reducing the head size 2) decrease the occurrence of DNA damage 3) remove the epigenetic markers that occur with histone modifications.
The structure of sperm head is also related to the protamine levels. The ratio of protamine 2 to protamine 1 and transition nuclear proteins has been found to change the sperm head shape in various species of Mus, the genus of mice, by altering the expression of protamine 2 via mutations in its promoter region. A decrease in the ratio has been found to increase the competitive ability of sperm in Mus species. However, further testing is required to determine how this ratio influences the shape of the head and whether monogamy has an impact on this selection. In humans, studies show that men who have imbalanced Prm1/Prm2 are subfertile or infertile.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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