TGM2-Mediated Histone Transglutamination is Dictated by Steric Accessibility

Histone monoaminylation is a new class of chromatin post-translational modification whereby glutamines are transamidated by tissue transglutaminase 2 (TGM2). Initial studies of these marks have identified serotonylation and dopaminylation on H3Q5, suggesting the modification is a marker of active chromatin. However, little work has been conducted to understand the regulation of TGM2-mediated monoaminylation of chromatin. Here, we employed a highly sensitive TGM2 assay using the small molecule biotin-cadaverine as an acyl-acceptor substrate. In doing so, we discovered that, in addition to H3Q5, H3Q19 and H2A.ZQ123 and Q124 are glutaminyl substrates for TGM2 in vitro. Modification of these residues occurs in a sequence independent manner, with the primary determining factor for substrate selectivity being steric accessibility. We demonstrated that moving substrate glutamines near the histone fold domain turns them into non-substrates. Furthermore, we found that steric accessibility of glutamines appears to be controlled by higher order chromatin structures. Using nucleosome arrays as substrates, we successfully showed that formation of chromatin fibers in vitro reduces levels of monoaminylation. We then successfully extended these findings to a cell based system, showing that TGM2 is not enriched in heterochromatin and that H3Q5 serotonylation and H3K9me3 are mutually exclusive marks. In total, our results strongly suggest that TGM2, and monoaminylation, are excluded from heterochromatin based on steric accessibility Adherent HeLa S3 cells were cultured in medium containing DMEM high glucose, 10% FBS and 1% P/S.