Modulation of alpha 5 beta 1 integrin functions by the phospholipid and cholesterol contents of cell membranes

Several modifications of the alpha 5 beta 1 integrin, which alter its intra cellular and extracellular interaction with fibronectin and other proteins, have been reported. However, the significance of the lateral mobility of i ntegrin molecules in the plasma membrane, as a regulator of their distribut ion and function, is poorly understood. We examined this problem by increas ing the cholesterol content of plasma membranes, and consequently modifying the fluidity of membrane phospholipids, in rat fibroblasts. Under these co nditions, the clustering of alpha 5 beta 1 integrin molecules in focal adhe sions, their adhesion to the cell-binding domain of fibronectin, and their association with the cytoskeletal protein talin were significantly enhanced as compared to control cells. However, the activation of MAP-kinase pathwa ys by the association of fibronectin with alpha 5 beta 1 integrin, End its association with integrin-linked kinase (ilk), were suppressed. The treated cells also showed distinct changes in shape, and their actin stress fiber network was more dense and thick as compared to control cells. The changes in fluidity of phospholipids occurred differentially and fluidity of phosph atidyl-ethanolamine increased, while that of phosphatidyl-choline was reduc ed. Our results suggest that proteins in focal adhesions could be partition ed in specific lipid domains, which regulate specific aspects of alpha 5 be ta 1 integrin functions. J. Cell. Biochem. 77:517-528, 2000. (C) 2000 Wiley -Liss, Inc.