<p>Uncropped Western blots from this study.</p>

MARCHF5 is an E3 ubiquitin ligase best known for regulating mitochondrial homeostasis and antiviral signaling, but not apoptosis. We thus investigated how MCL1 and MARCHF5 cooperate to promote PEL cell survival. CRISPR knockout (KO) of <i>MARCHF5</i> in PEL cell lines resulted in a significant increase in apoptosis despite the presence of MCL1.

The anti-apoptotic function of MARCHF5 was dependent on its E3 ligase and dimerization activities. Loss of MARCHF5 or inhibition of the 26S proteasome furthermore stabilized the MCL1 antagonist NOXA without affecting levels of MCL1. Interestingly, <i>NOXA</i> KO provides a fitness advantage to PEL cells suggesting that NOXA is the pro-apoptotic signal that necessitates the anti-apoptotic activities of MCL1 and MARCHF5.

Endogenous reciprocal co-immunoprecipitation experiments show that MARCHF5 and NOXA are found in the same protein complex. Finally, <i>NOXA</i> KO rescued the loss of cell viability due to <i>MARCHF5</i> KO. Our findings thus provide the mechanistic link that underlies the genetic interaction between MCL1 and MARCHF5.

We propose that MARCHF5 induces the degradation of the MCL1 antagonist NOXA thus reinforcing the pro-survival role of MCL1 in these tumor cells. This newly appreciated interaction of the MCL1 and MARCHF5 oncogenes may be useful to improve the design of combination therapies for KSHV malignancies.</p></div>