Direct histone proteoform profiling of the unannotated, endangered coral <i>Acropora cervicornis</i>

Epigenetic modifications directly regulate the patterns of gene expression by altering DNA accessibility and chromatin structure. A knowledge gap is presented by the need to directly measure these modifications, especially for unannotated organisms with unknown primary histone sequences. In the present work, we developed and applied a novel workflow for identifying and annotating histone proteoforms directly from mass spectrometry-based measurements for the endangered Caribbean coral Acropora cervicornis. Combining high-accuracy de novo top-down and bottom-up analysis based on tandem liquid chromatography, trapped ion mobility spectrometry, non-ergodic electron-based fragmentation, and high-resolution mass spectrometry, near complete primary sequence (up to 99%) and over 86 post-translational modification annotations were obtained from pull-down histone fractions. In the absence of reliable genome annotations, H2A, H2B, and H4 histone sequences and the annotation of the post-translational modifications of the stressed A. cervicornis coral allow for a better understanding of chromatin remodeling and new strategies for targeting intervention and restoration of endangered reef corals.