α2β1 integrin controls association of Rac with the membrane and triggers quiescence of endothelial cells

Integrin receptors and their extracellular matrix ligands provide cues to cell proliferation, survival, differentiation and migration. Here, we show that α2β1 integrin, when ligated to the basement membrane component laminin-1, triggers a proliferation arrest in primary endothelial cells. Indeed, in the presence of strong growth signals supplied by growth factors and fibronectin, α2β1 engagement alters assembly of mature focal adhesions by α5β1 and leads to impairment of downstream signaling and cell-cycle arrest in the G1 phase. Although the capacity of α5β1 to signal for GTP loading of Rac is preserved, the joint engagement of α2β1 interferes with membrane anchorage of Rac. Adapting the ‘split-ubiquitin’ sensor to screen for membrane-proximal α2 integrin partners, we identified the CD9 tetraspanin and further establish its requirement for destabilization of focal adhesions, control of Rac subcellular localization and growth arrest induced by α2β1 integrin. Altogether, our data establish that α2β1 integrin controls endothelial cell commitment towards quiescence by triggering a CD9-dependent dominant signaling.