Mutations in epigenetic pathways are common oncogenic drivers. Histones, the fundamental substrate for chromatin-modifying and remodeling enzymes, are mutated in tumors including in gliomas, sarcomas, head and neck cancers, and carcinosarcomas. Classical 'oncohistone' mutations occur in the N-terminal tail of histone H3 and impact the function of Polycomb Repressor Complexes 1 and 2. However, the prevalence and function of histone mutations in additional tumor contexts is unknown. Here we show that somatic histone mutations conservatively occur in ~ 4% of tumors of diverse types and in critical regions of histone proteins. Mutations occur in all four core histones, in both the N-terminal tails and globular histone fold domains, and at or near residues that harbor important post-translational modifications. Many globular domain mutations are either homologous to yeast mutants that abrogate the need for SWI/SNF function, occur in the key regulatory 'acidic patch' of histone H2A and H2B, or are predicted to disrupt the H2B-H4 interface. The histone mutation dataset (https://bit.ly/2GXH5Ve) and the hypotheses presented herein on the impact of the mutations on important chromatin functions should serve as a resource and starting point for the chromatin and cancer biology fields in exploring an expanding role of histone mutations in cancer.

Methionine aminopeptidase (MetAP) catalyzes the hydrolytic cleavage of the N-terminal methionine from newly synthesized polypeptides. The extent of methionyl removal from a protein is dictated by its N-terminal peptide sequence. Earlier studies revealed that MetAPs require amino acids containing small side chains (e.g., Gly, Ala, Ser, Cys, Pro, Thr, and Val) as the P1' residue, but their specificity at positions P2' and beyond remains incompletely defined. In this work, the substrate specificities of Escherichia coli MetAP1, human MetAP1, and human MetAP2 were systematically profiled by screening against a combinatorial peptide library and kinetic analysis of individually synthesized peptide substrates. Our results show that although all three enzymes require small residues at the P1' position, they have differential tolerance for Val and Thr at this position. The catalytic activity of human MetAP2 toward Met-Val peptides is consistently two orders of magnitude higher than that of MetAP1, suggesting that MetAP2 is responsible for processing proteins containing N-terminal Met-Val and Met-Thr sequences in vivo. At positions P2' to P5', all three MetAPs have broad specificity, but are poorly active toward peptides containing a proline at the P2' position. In addition, the human MetAPs disfavor acidic residues at the P2' to P5' positions. The specificity data have allowed us to formulate a simple set of rules that can reliably predict the N-terminal processing of E. coli and human proteins. KeywordsMethionine aminopeptidase; N-terminal processing; substrate specificity; kinetics; peptide library Ribosomal protein synthesis is universally initiated with methionine (in eukaryotic cytoplasm) or N-formylmethionine (in prokaryotes, mitochondria, and chloroplasts). During protein maturation, the N-formyl group is removed by peptide deformylase, leaving methionine with a free NH 2 group (1). Subsequently, the initiator methionine is removed from many but not all of the proteins by methionine aminopeptidase(s) (MetAPs). For example, in a cytosolic extract of Escherichia coli, only 40% of the polypeptides retain the initiator methionine, whereas the majority of the polypeptides display alanine, serine, or threonine at their N-termini (2). There are two types of MetAPs, MetAP1 and MetAP2.

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