| Significantly , both overexpression and siRNA knock down approaches , and analysis of cells devoid of PP4c , unequivocally show that HDAC 3 activity is inversely proportional to the cellular abundance of PP 4 ( c ) . ^^^ Interestingly , unlike other class 1 HDACs , HDAC 3 uniquely copurifies with the catalytic and regulatory subunits of the protein serine / threonine phosphatase 4 complex ( PP4c / PP4R1 ) . ^^^ Furthermore , HDAC 3 complexes displayed protein phosphatase activity and a series of subsequent mutational analyses revealed that the N terminus of HDAC 3 ( residues 1 122 ) was both necessary and sufficient for HDAC 3 PP4c interactions . ^^^ |