Structure of Tetrahymena GCN5 bound to coenzyme A and a histone H3 peptide

Gene activation is a highly regulated process that requires the coordinated action of proteins to relieve chromatin repression and to promote transcri ptional activation. Nuclear histone acetyltransferase (HAT) enzymes provide a mechanistic link between chromatin destabilization and gene activation b y acetylating the E-amino group of specific lysine residues within the amin oterminal tails of core histones to facilitate access to DNA by transcripti onal activators(1,2). Here we report the high-resolution crystal structure of the HAT domain of Tetrahymena GCN5 (tGCN5) bound with both its physiolog ically relevant ligands, coenzyme A (CoA) and a histone H3 peptide, and the structures of nascent tGCN5 and a tGCN5/acetyl-CoA complex. Our structural data reveal histone-binding specificity for a random-coil structure Contai ning a G-K-X-P recognition sequence, and show that CoA is essential for reo rienting the enzyme for histone binding. Catalysis appears to involve water -mediated proton extraction from the substrate lysine by a glutamic acid ge neral base and a backbone amide that stabilizes the transition-state reacti on intermediate. Comparison with related N-acetyltransferases indicates a c onserved structural framework for CoA binding and catalysis, and structural variability in regions associated with substrate-specific binding.