Ubiquitin-independent degradation of antiapoptotic MCL-1

Antiapoptotic myeloid cell leukemia 1 (MCL-1) is an essential modulator of survival during the development and maintenance of a variety of cell lineages. Its turnover, believed to be mediated by the ubiquitin-proteasome system, facilitates apoptosis induction in response to cellular stress. To investigate the contribution of ubiquitinylation in regulating murine MCL-1 turnover, we generated an MCL-1 mutant lacking the lysine residues required for ubiquitinylation (MCL-1(KR)). Here, we demonstrate that despite failing to be ubiquitinylated, the MCL-1(KR) protein is eliminated at a rate similar to that of wild-type MCL-1 under basal and stressed conditions. Moreover, the degradation of wild-type MCL-1 is not affected when ubiquitin-activating enzyme E1 activity is blocked. Likewise, both wild-type and MCL-1(KR) proteins are similarly degraded when expressed in primary lymphocytes. Supporting these findings, unmodified, in vitro-translated MCL-1 can be degraded in a cell-free system by the 20S proteasome. Taken together, these data demonstrate that MCL-1 degradation can occur independently of ubiquitinylation.