HP1 binding to native chromatin in vitro is determined by the hinge region and not by the chromodomain

Abstract

We have isolated the complete coding sequences for two Xenopus laevis isoforms of heterochromatin protein 1, corresponding to HP1α and HP1γ. The sequence of xHP1α shows considerable divergence from its mammalian homologues, whereas xHP1γ is highly conserved. Functionally, xHP1α behaves identically to human HP1α. We observe unexpected differences between the two HP1 variants in binding native soluble chromatin, which seem to correlate with their distinct nuclear distributions in vivo. A surprising finding is that the characteristic interaction of HP1 chromodomains with histone H3 at methylated lysine 9 is not detected in preformed chromatin due to its inaccessibility. Instead, we localize a strong chromatin-binding activity to the short hinge region between the chromodomain and the chromoshadow domain of xHP1α but not xHP1γ. This novel chromatin-binding activity has a non-specific DNA-binding component in addition to a linker histone-dependent preference for an altered chromatin structure with a likely heterochromatin organization.