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J Cell Biol. 2015 Nov 23;211(4):863-79. doi: 10.1083/jcb.201501072.

PAK4 promotes kinase-independent stabilization of RhoU to modulate cell adhesion.

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Division of Cancer Studies, King's College London, London SE1 1UL, England, UK.
Tumour Biology and Metastasis, Cancer Research UK Cancer Therapeutics Unit, The Institute of Cancer Research, London SM2 5NG, England, UK.
Breast Research Pathology, Department of Research Oncology, Division of Cancer Studies, School of Medicine, Guy's Hospital, King's College London, London SE1 9RT, England, UK.
Division of Cancer Studies, King's College London, London SE1 1UL, England, UK


P21-activated kinase 4 (PAK4) is a Cdc42 effector protein thought to regulate cell adhesion disassembly in a kinase-dependent manner. We found that PAK4 expression is significantly higher in high-grade human breast cancer patient samples, whereas depletion of PAK4 modifies cell adhesion dynamics of breast cancer cells. Surprisingly, systematic analysis of PAK4 functionality revealed that PAK4-driven adhesion turnover is neither dependent on Cdc42 binding nor kinase activity. Rather, reduced expression of PAK4 leads to a concomitant loss of RhoU expression. We report that RhoU is targeted for ubiquitination by the Rab40A-Cullin 5 complex and demonstrate that PAK4 protects RhoU from ubiquitination in a kinase-independent manner. Overexpression of RhoU rescues the PAK4 depletion phenotype, whereas loss of RhoU expression reduces cell adhesion turnover and migration. These data support a new kinase-independent mechanism for PAK4 function, where an important role of PAK4 in cellular adhesions is to stabilize RhoU protein levels. Thus, PAK4 and RhoU cooperate to drive adhesion turnover and promote cell migration.

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