Functional divergence and structural changes of Class IV histone deacetylases (HDACs) across the tree of life.

Class IV histone deacetylases (HDACs) are the least understood branch of the classical zinc-dependent HDAC family with HDAC11 standing out as the sole member of Class IV HDACs. Using a broad phylogenetic dataset spanning bacteria, archaea, and eukaryotes, we identified two deeply conserved HDAC11 lineages, clades A and B, that differ in evolutionary origin, predicted subcellular localization, and enzymatic properties. Clade A is enriched in phototrophic eukaryotes and targeted to mitochondria or plastids, whereas clade B predominates in heterotrophs and localizes mainly to the cytoplasm or nucleus. High-resolution crystal structures of selected representatives from each clade revealed a conserved catalytic core but distinct structural features-including electrostatic surface profiles, loop architectures, and foot pocket geometries-that clearly separate the two lineages and act as sequential "selectivity filters" shaping substrate specificity. Biochemical assays show robust long-chain fatty acid deacylase activity in clade B enzymes, but no detectable activity for any of clade A representatives against peptide substrates, suggesting adaptation to alternative, nonpeptidic targets. Together, these findings define a revised evolutionary framework for HDAC11 and provide structural and functional insights into the diversification of this ancient enzyme family.