The retinoic acid-induced differentiation of F-9 teratocarcinoma cells in monolayer culture is accompanied by the accumulation of fibrillar fibronectin deposits, the appearance of a highly structured actin cytoskeleton, and the redistribution of integrin to apparent sites of substrate contact. We have studied the 140-kD fibronectin receptor during this process and report that although the integrin molecule is present in equivalent amounts before and after differentiation, the level of integrin phosphorylation decreases dramatically as the cells differentiate. This loss of phosphorylation coincides temporally with the observed changes in actin, fibronectin, and integrin organization. The phosphorylation state of integrin thus may mediate developmentally regulated cell-matrix interactions.

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