Puma(*)Mcl-1 interaction is not sufficient to prevent rapid degradation of Mcl-1. 
Although Puma (p53 upregulated modulator of apoptosis) was known as a principal mediator of cell death in response to diverse apoptotic signals, the molecular mechanism underlying its proapoptotic regulation remains largely uncharacterized.  Here we reported that myeloid cell leukemia-1 (Mcl-1), an anti-apoptotic member of the Bcl-2 family with a rapid turnover rate, interacts with Puma.  The Puma/Mcl-1 interaction was verified by both yeast two-hybrid assay and co-immuno-precipation studies.  Their binding sites were mapped to BH3 (Bcl-2 homology) domain of Puma and BH1 domain of Mcl-1, respectively.  Mcl-1 and Puma was shown to colocalize at the mitochondria by immunostaining.  The level of Mcl-1 was increased when coexpressed with Puma, indicating Puma is able to stabilize Mcl-1.  Puma binding to Mcl-1 via its BH3 domain is the prerequisite for this effect, which is further supported by the finding that Puma mutant lacking BH3 domain no longer promotes Mcl-1 protein stability.  This Puma-enhanced Mcl-1 stabilization was validated in vivo under non-overexpression conditions.  We also showed that BH1 domain is essential for Mcl-1 to inhibit Puma-induced apoptosis, since Mcl-1 mutant lacking BH1 domain completely abrogates its protective function.  In addition, we concluded that binding of Puma to BH1 domain of Mcl-1 is necessary, but not sufficient to prevent rapid degradation of Mcl-1.  In addition to PEST (proline, glutamic acid, serine, and threonine) and BH1 domain, some additional degradation signal is expected to reside in the C-terminal region of Mcl-1.  In conclusion, our results provide the first evidence that the interaction between Mcl-1 and Puma may represent a novel mechanism by which Mcl-1 prevents apoptosis by increasing its stability through binding to Puma.