Dynamics of the HP1β-PCNA-containing complexes in DNA replication and repair

Heterochromatin protein 1 (HP1), a small non-histone chromosomal protein, was recently shown to form a complex in vivo with Proliferating Cell Nuclear Antigen (PCNA), a key factor in DNA replication. The complex, which requires HP1 beta in a form of a dimer, is engaged in DNA repair and replication. We now provide evidence based on FRET-FLIM live cell studies confirming the association and close proximity between HP1 beta and PCNA in the complex. We also demonstrate using FRAP, that although HP1 beta-PCNA complexes are highly mobile in nonreplicating nuclei, when engaged in DNA replication, they become bound and do not exchange with the mobile pool. These observations are in agreement with a notion that a subpopulation of HP1 molecules interact with PCNA in vivo during DNA replication. Similarly, HP1 beta which is associated with PCNA in regions of DNA repair, is bound and does not exchange with the mobile pool, suggesting that HP1 beta in association with PCNA may be a component of a DNA repair complex.