Differentiation-specific association of HP1 alpha and HP1 beta with chromocentres is correlated with clustering of TIF1 beta at these sites
Mammalian heterochromatin protein 1 (HP1 alpha, HP1 beta, HP1 gamma subtypes) and transcriptional intermediary factor TIF1 beta play an important role in the regulation of chromatin structure and function. Here, we investigated the nuclear arrangement of these proteins during differentiation of embryonal carcinoma P19 cells into primitive endoderm and into the neural pathway. Additionally, the differentiation potential of trichostatin A (TSA) and 5-deoxyazacytidine (5-dAzaC) was studied. In 70% of the cells from the neural pathway and in 20% of TSA-stimulated cells, HP1 alpha and HP1 beta co-localized and associated with chromocentres (clusters of centromeres), which correlated with clustering of TIF1 beta at these heterochromatic regions. The cell types that we studied were also characterized by a pronounced focal distribution of HP1 gamma. The above-mentioned nuclear patterns of HP1 and TIF1 beta proteins were completely different from the nuclear patterns observed in the remaining cell types investigated, in which HP1 alpha was associated with chromocentres while HP1 beta and HP1 gamma were largely localized in distinct nuclear regions. Moreover, a dispersed nuclear distribution of TIF1 beta was observed. Our findings showed that the nuclear arrangement of HP1 subtypes and TIF1 beta is differentiation specific, and seems to be more important than changes in the levels of these proteins, which were relatively stable during all the induced differentiation processes.