RSU-1 interaction with prohibitin-2 links cell-extracellular matrix detachment to down-regulation of extracellular signal-regulated kinase signalling.

Cell-extracellular matrix (ECM) detachment is known to down-regulate ERK signalling, an intracellular pathway that is central for control of cell behaviour. How cell-ECM detachment is linked to downregulation of ERK signalling, however, is incompletely understood. We show here that focal adhesion protein Ras Suppressor 1 (RSU1) plays a critical role in cell-ECM detachment induced down-regulation of ERK signalling. We have identified prohibitin 2 (PHB2), a component of membrane lipid rafts, as a novel binding protein of RSU1, and mapped a major RSU1-binding site to PHB2 amino acids 150-206 in the C-terminal region of the PHB/SPFH (stomatin/prohibitin/flotillin/HflKC) domain. The PHB2-binding is mediated by multiple sites located in the N-terminal leucine-rich repeat (LRR) region of RSU-1. Depletion of PHB2 suppressed cell-ECM adhesion induced ERK activation. Furthermore, cell-ECM detachment increased RSU1 association with membrane lipid rafts and interaction with PHB2. Finally, knockout of RSU1 or inhibition of RSU1 interaction with PHB2 by overexpression of the major RSU1-binding PHB2 fragment (amino acids 150-206) effectively suppressed the cell-ECM detachment induced down-regulation of ERK signalling. Expression of venus-tagged wild-type RSU1, but not that of venus-tagged PHB2-binding defective RSU1 mutant in which the N-terminal LRR region is deleted, restored cell-ECM detachment induced down-regulation of ERK signalling. Our results identify a novel RSU1-PHB2 signalling axis that senses cell-ECM detachment and links it to down-regulation of ERK signalling.