Investigating The Role Of Histone H2a Proteolysis During Stem Cell Differentiation

Histone proteolysis is a poorly understood phenomenon in which the N-terminal tails of histones are irreversible cleaved by intracellular proteases. During development, histone post-translational modifications are known to orchestrate gene expression patterns that ultimately drive cell fate decisions. Therefore, deciphering the mechanisms of histone proteolysis is necessary to enhance the understanding of cellular differentiation. Here we show that H2A is cleaved by the lysosomal protease Cathepsin L during ESCs differentiation. Using quantitative mass spectrometry (MS), we identified L23 to be the primary cleavage site that gives rise to the clipped form of H2A (cH2A), which reaches a maximum level of ~1% of total H2A after 4 days of differentiation. Using ChIP-seq, we found that preventing proteolysis leads to an increase in acetylated H2A at promoter regions in differentiated ES cells. We also report novel readers of acetylated H2A in pluripotent ES cells, including members of the pBAF complex, which recognize distinct forms of H2A acetylation. Finally, we show that H2A proteolysis abolishes this recognition. Altogether, our data suggest that proteolysis serves as an efficient mechanism to silence pluripotency genes and destabilize the nucleosome core particle.