Posttranslational modifications of the HP1-homologes HcpA and HcpB in Dictyostelium discoideum: functional role of a highly conserved Lysin in the chromo-shadow domain.

The Heterochromatin Protein 1 (HP1) is an important factor for the establishment and maintenance of heterochromatin. Here we show, that the Dictyostelium discoideum homologes HcpA and HcpB are decorated with post-translational modifications (PTMs) namely methylation and phosphorylation. Modification of a highly conserved lysine in the chromo-shadow domain of both HcpA (K203R) and HcpB (K222ac) appears to play an essential role in growth, fruiting body formation and spore shape since a HcpA (K203R) mutation specifically affects these features. Presumably, the effect is caused by alterations in Hcp homo- and heterodimer formation. The assumption is supported by tethering experiments: targeting wt-HcpA to a GFP reporter gene results in efficient silencing while the HcpA (K203R) mutant fails to repress GFP expression.

Our data describe the first evidence for the functional importance of this posttranslational modification in HP1.