Phosphorylation of a group of high molecular weight basic nuclear proteins during spermatogenesis in the winter...

A set of high molecular weight basic nuclear proteins makes up 25% by weight of the acid-soluble chromatin proteins in the mature sperm of the winter flounder (Pseudopleuronectes americanus). When these proteins first appear in testis cell nuclei in late October they are extensively phosphorylated, with an average phosphate content of 0.15 mg/mg of protein. As a result of this modification the proteins precipitate at pH 7.0. Their serine residues show enhanced susceptibility to beta-elimination, and partial acid hydrolysis of the proteins releases O-phosphoserine. The high serine content of these proteins (22 mol %) is sufficient to accommodate all of the bound phosphate as O-phosphoserine. At this point in spermatogenesis almost half of the histones H2A and H4 are also phosphorylated. As spermatogenesis proceeds there is a wave of dephosphorylation in which phospo-H2A and phospho-H4 disappear and the bulk of the phosphate is removed from the high molecular weight proteins T and H6 are lost. By January the high molecular weight basic nuclear proteins are completely dephosphorylated and differential salt extraction of the sperm chromatin shows that these proteins are more tightly bound than H1 and are eluted with the nucleosomal core histones at a salt concentration of greater 1 M.