Plasticity of HP1 proteins in mammalian cells

We have compared the distribution of endogenous heterochromatin protein 1 ( HP1) proteins (alpha, beta and gamma) in different epithelial lines, pluripotent stem cells and embryonic fibroblasts. In parallel, we have interrogated assembly and dynamics of newly expressed HP1-GFP proteins in cells lacking both HP1 alpha and HP1 beta alleles, blocked at the G1-S boundary, or cultured in the presence of HDAC and HAT inhibitors. The results reveal a range of cell type and differentiation state-specific patterns that do not correlate with 'fast' or 'slow' subunit exchange in heterochromatin. Furthermore, our observations show that targeting of HP1 gamma to heterochromatic sites depends on HP1 alpha and H1 beta and that, on an architectural level, HP1 alpha is the most polymorphic variant of the HP1 family. These data provide evidence for HP1 plasticity under shifting microenvironmental conditions and offer a new conceptual framework for understanding chromatin dynamics at the molecular level.