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 alpha 2 beta 1 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, alpha 2 beta 1 engagement alters assembly of mature focal adhesions by alpha 5 beta 1 and leads to impairment of downstream signaling and cell-cycle arrest in the G1 phase. Although the capacity of alpha 5 beta 1 to signal for GTP loading of Rac is preserved, the joint engagement of alpha 2 beta 1 interferes with membrane anchorage of Rac. Adapting the 'split-ubiquitin' sensor to screen for membrane-proximal alpha 2 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 alpha 2 beta 1 integrin. Altogether, our data establish that alpha 2 beta 1 integrin controls endothelial cell commitment towards quiescence by triggering a CD9-dependent dominant signaling.