REQUIREMENT FOR RHO IN INTEGRIN SIGNALING

Overnight culture of Swiss 3T3 cells in serum-free medium leads to los s of focal adhesions and associated actin stress fibres, although the cells remain well spread. The small GTP-binding protein Rho is require d for the formation of stress fibres and focal adhesions induced by gr owth factors such as lysophosphatidic acid (LPA) in serum-starved Swis s 3T3 cells, and for the LPA-induced tyrosine phosphorylation of sever al focal adhesion proteins. Plating of cells on extracellular matrix p roteins also stimulates protein tyrosine phosphorylation and the forma tion of stress fibres and focal adhesions in the absence of added grow th factors. These responses were inhibited in cells scrape-loaded with the Rho inhibitor C3 transferase. Focal adhesion and stress fibre for mation was also triggered by addition of a peptide GRGDS, which is rec ognised by a number of integrins and is contained within the cell bind ing domain of a variety of extracellular matrix proteins. The activity of the GRGDS peptide was blocked by microinjecting cells with C3 tran sferase, suggesting that peptide binding to integrins stimulates a Rho -dependent assembly of focal adhesions. These experiments indicate tha t Rho is involved in signalling downstream of integrins.