Epigenetic marking of sperm by post-translational modification of histones and protamines

Background: The concept that individual traits can be acquired and transmitted by the germline through epigenetic mechanisms has gained recognition in the past years. However, epigenetic marks in sperm have not been are not well identified. Results: Using a novel proteomic approach that combines peptide-based bottom-up and intact protein top-down tandem mass spectrometry, we report the identification of epigenetic marks on histones and protamines in adult mouse sperm. We identified a total of 26 post-translational modifications (PTMs) on specific residues of the core histones H2B, H3 and H4, and the linker histone H1, four of which had not been described previously in any tissue or cell line. We also detected 11 novel PTMs on the protamines PRM1 and PRM2 and observed that they are present in specific combinations on individual protamines. Conclusions: Both histones and protamines carry multiple PTMs in the adult mouse sperm. On protamines, specific PTM combinations might form a 'protamine code' similar to the 'histone code'. These findings suggest a potential role for PTMs on sperm histones and protamines in epigenetic signatures underlying transgenerational inheritance.