Regulation of apoptosis and cell cycle progression by MCL1; Differential role of proliferating cell nuclear antigen

MCL1 (ML1 myeloid cell leukemia 1), a Bcl-2 (B- cell lyphoma-leukemia 2) homologue, is known to function as an anti-apoptotic protein. Here we show in vitro and in vivo that MCL1 interacts with the cell cycle regulator, proliferating cell nuclear antigen (PCNA). This finding prompted us to investigate whether MCL1, in addition to its anti-apoptotic function, has an effect on cell cycle progression. A bromodeoxyuridine uptake assay showed that the overexpression of MCL1 significantly inhibited the cell cycle progression through the S-phase. The S-phase of the cell cycle is also known to be regulated by PCNA. A mutant of MCL1 that lacks PCNA binding (MCL1_Δ4A) could not inhibit cell cycle progression as effectively as wild type MCL1. In contrast, MCL1_Δ4A retained its anti-apoptotic function in HeLa cells when challenged by Etoposide. In addition, the intracellular localization of MCL1_Δ4A was identical to that of wild type MCL1. An in vitro pull-down assay suggested that MCL1 is the only Bc1-2 family protein to interact with PCNA. In fact, MCL1, not other Bc1-2 family proteins, contained the PCNA-binding motif described previously. Taken together, MCL1 is a regulator of both apoptosis and cell cycle progression, and the cell cycle regulatory function of MCL1 is mediated through its interaction with PCNA.