Accessibility of the genome to DNA-binding transcription factors is regulated by 
proteins that control the acetylation of amino-terminal lysine residues on 
nucleosomal histones. Specifically, histone deacetylase (HDAC) proteins repress 
transcription by deacetylating histones. To date, the only known regulatory 
mechanism of HDAC1 function is via interaction with associated proteins. 
Although the control of HDAC1 function by protein interaction and recruitment is 
well precedented, we were interested in exploring HDAC1 regulation by 
post-translational modification. Human HDAC1 protein was analyzed by ion trap 
mass spectrometry, and two phosphorylated serine residues, Ser(421) and 
Ser(423), were unambiguously identified. Loss of phosphorylation at Ser(421) and 
Ser(423) due to mutation to alanine or disruption of the casein kinase 2 
consensus sequence directing phosphorylation reduced the enzymatic activity and 
complex formation of HDAC1. Deletion of the highly charged carboxyl-terminal 
region of HDAC1 also decreased its deacetylase activity and protein 
associations, revealing its requirement in maintaining HDAC1 function. Our 
results reinforce the importance of protein associations in modulating HDAC1 
function and provide the first step toward characterizing the role of 
post-translational modifications in regulating HDAC activity in vivo.