Do Histone Variants Play a Role in Spermatid Specific Chromatin Remodeling

Another strong argument for a spermatid-specific histone code is the presence of core histone variants in late spermiogenesis. Most, if not all, of the post-translationally modified residues of N- and C-terminal parts of H3, H2A and H2B are conserved in H3.3, TH2A and TH2B. These residues have therefore the potential to be modified in the same way as in somatic histones. However, the phosphorylable S14 of H2B is replaced by phenylalanine in TH2B. This poorly characterized modification has recently been reported to target H2B in apoptotic chromatin (Cheung et al. 2003). TH2B will therefore escape this particular signalization.

Very interestingly, S19 of H2A is also replaced by a phenylalanine in TH2A. This suggests, first, that H2A S19 could also be a site of phosphorylation, and, second, that the hydrophobic phenylalanine residue could create a special signal in the N-terminal part of TH2A and TH2B for the interaction with specific factors in spermatids. Furthermore, the important variations in the N-termi-nal tail part of TH2B could modify the recognition of these amino acids by histone-modifying enzymes. For instance, it could accelerate or inhibit the modification of a specific residue.

Finally, sequence divergence observed in the histone fold regions of testis-specific histones compared to the somatic types may have dramatic effects on the nucleosome stability and facilitate histone replacement in elongating spermatids (Fan et al. 2002).

0 0

Post a comment