Phospho-HP1γ (Ser83) Antibody #2600

Heterochromatin protein 1 (HP1) is a family of heterochromatic adaptor molecules involved in both gene silencing and higher order chromatin structure (1). All three HP1 family members (α, β, and γ) are primarily associated with centromeric heterochromatin; however, HP1β and γ also localize to euchromatic sites in the genome (2,3). HP1 proteins are approximately 25 kDa in size and contain a conserved amino-terminal chromodomain, followed by a variable hinge region and a conserved carboxy-terminal chromoshadow domain. The chromodomain facilitates binding to histone H3 tri-methylated at Lys9, a histone "mark" closely associated with centromeric heterochromatin (4,5). The variable hinge region binds both RNA and DNA in a sequence-independent manner (6). The chromoshadow domain mediates the dimerization of HP1 proteins, in addition to binding multiple proteins implicated in gene silencing and heterochromatin formation, including the SUV39H histone methyltransferase, the DNMT1 and DNMT3a DNA methyltransferases, and the p150 subunit of chromatin assembly factor 1 (CAF-1) (7-9). In addition to contributing to heterochromatin formation and propagation, HP1 and SUV39H1 are also found complexed with retinoblastoma (Rb) and E2F6 proteins, both of which function to repress euchromatic gene transcription in quiescent cells (10,11). HP1 proteins are subject to multiple types of post-translational modifications, including phosphorylation, acetylation, methylation, ubiquitination, and sumoylation, suggesting multiple means of regulation (12-14).

HP1γ is phosphorylated on Ser83 by protein kinase A (PKA) in vitro, and activation of PKA by forskolin and IBMX treatment leads to increased phosphorylation in vivo (14). Phosphorylation of HP1γ on Ser83 also increases during mitosis as demonstrated by the Phospho-HP1γ (Ser83) Antibody, which shows increased immunofluorescent staining in untreated mitotic cells and increased Western blot signal in lysates from cells arrested in mitosis by treatment with paclitaxel. Phosphorylation of Ser83 only occurs on a subpopulation of HP1γ found associated with euchromatin, specifically HP1γ bound to coding regions of active genes (14). 
This phosphorylation impairs the ability of HP1γ to silence transcription and may be a marker for transcription elongation (14).