Cooperative regulation of YAP activity by STK40 and CDK7 in cancer progression

Yes-associated protein (YAP) is a mechanosensitive transcriptional co-activatorthat drives cell proliferation and tumorigenesis. We previously identified thepseudokinase STK40 as a scaffold that directly binds and post-translationallyregulates YAP, with STK40 knockdown reducing YAP abundance and nuclearlocalization. In hormone receptor positive breast cancer, resistance to CDK4/6inhibitors remains a major challenge, and emerging evidence from ourcollaborative studies suggests that CDK7 transcriptionally regulates YAPthrough a CDK7-RNAPII-YAP-CDK6 axis contributing to therapeutic resistance.Whether STK40 and CDK7 function independently or converge on a sharedmechanism remains unknown. Using SAS cells, we found that CDK7knockdown reduced YAP protein levels in both cytosol and nucleus anddecreased the nuclear-to-cytosolic ratio, whereas dual CDK7 and STK40knockdown phenocopied CDK7 loss alone. CDK7 overexpression partiallyrescued YAP depletion caused by STK40 knockdown, indicating that CDK7acts downstream of STK40 in the same regulatory pathway. Proximity ligationassays confirmed a direct STK40 and YAP interaction. Reducing actomyosintension caused YAP to relocalize to the plasma membrane, but thisredistribution was abolished by STK40 knockdown. ROCK inhibition enhancedYAP Y357 phosphorylation at the plasma membrane, a modification requiredfor nuclear entry, yet this phosphorylation was completely lost in STK40-depleted cells, indicating that STK40 is required for positioning YAP at themembrane to initiate mechanotransduction driven nuclear import. In contrast,CDK7 knockdown did not prevent YAP membrane recruitment but caused YAPto accumulate at the nuclear envelope without entering the nucleus, suggestingthat CDK7 regulates a downstream step essential for nuclear translocation.Together, these data reveal a previously unrecognized STK40 and CDK7regulatory mechanism that cooperatively controls YAP abundance andsubcellular distribution. STK40 directs YAP to the plasma membrane for Y357phosphorylation, whereas CDK7 acts as a critical checkpoint enabling tension-dependent nuclear import. This mechanism represents a potential therapeuticvulnerability in YAP-driven cancers, including CDK4/6 inhibitor-resistant HR+breast cancer.