The stress-sensing MARCH5 axis governs blood cancer apoptotic resilience via tractable protein-protein interactions.

The mitochondrial E3 ligase MARCH5 has consistently emerged as a dependency in unbiased screens in acute myeloid leukemia and myeloma, yet the underpinning mechanism remains ill-defined. Here, we show that MARCH5 cooperates with UBE2J2 and MFN2, forming a stress-sensing complex at mitochondria-ER contact sites (MERCS) that restrains apoptosis in response to diverse organellar damage signals. Loss of MARCH5 potently sensitizes diverse blood cancer cell lines to BCL-2 and BCL-XL inhibition and compromises stress tolerance. By contrast, non-hematopoietic cell lines exhibit a phenotype largely restricted to BCL-XL dependence, permitting tissue-selective therapeutic synergy with venetoclax and other agents. Mechanistically, spatial organization underpins this specificity. The complex assembles at MERCS, where it co-localizes with BCL-2 and BCL-XL but not MCL-1. Upon organellar damage, it dissociates prior to BAX/BAK activation, lowering the apoptotic threshold and enforcing reliance on neighboring BCL-2 and BCL-XL. Consistent with its distribution, MARCH5 loss minimally alters MCL-1 dependence, revealing a spatially encoded mechanism integrating diverse stress signals into cell-death decisions. To guide future therapeutics, we demonstrate that disrupting key protein-protein interactions within this complex is sufficient to sensitize blood cancer cell lines, restoring venetoclax responsiveness and prolonging survival in a murine model of refractory lymphoma. Genetic deletion of MARCH5 or UBE2J2 restored BH3-mimetic sensitivity to primary chronic lymphocytic leukemia cells rendered resistant by cytokine stimulation. These findings establish the MERCS-resident MARCH5 complex as a central regulator of malignant cell stress tolerance and highlight tractable protein interfaces for therapeutic targeting.