0929-8665/11 $58.00+.00 © 2011 Bentham Science Publishers Ltd. Protamines: Structural Complexity, Evolution and Chromatin Patterning

Abstract: Despite their relatively arginine-rich composition, protamines exhibit a high degree of structural variation. In-deed, the primary structure of these histone H1-related sperm nuclear basic proteins (SNBPs) is not random and is the de-pository of important phylogenetic information. This appears to be the result of their fast rate of evolution driven by posi-tive selection. The way by which the protein variability participates in the transitions that lead to the final highly con-densed chromatin organization of spermatozoa at the end of spermiogenesis is not clearly understood. In this paper we fo-cus on the transient chromatin/nucleoplasm patterning that occurs in either a lamellar step or an inversion step during early and mid-spermiogenesis. This takes place in a small subset of protamines in internally fertilizing species of verte-brates, invertebrates and plants. It involves “complex ” protamines that are processed, replaced, or undergo side chain modification (such as phosphorylation or disulfide bond formation) during the histone-to-protamine transition. Character-istic features of such patterning, as observed in TEM photomicrographs, include: constancy of ’ the dominant pattern re-peat distance m despite dynamic changes in developmental morphology, bicontinuity of chromatin and nucleoplasm, and chromatin orientation either perpendicular or parallel to the nuclear envelope. This supports the hypothesis that liquid-liquid phase separation by the mechanism of spinodal decomposition may be occurring during spermiogenesis in these species. Spinodal decomposition involves long wave fluctuations of the local concentration with a low energy barrier and thus differs from the mechanism of nucleation and growth that is known to occur during spermiogenesis in internally fer