Cytohesin-1 regulates beta-2 integrin-mediated adhesion through both ARF-GEF function and interaction with LFA-1

Intracellular signaling pathways, which regulate the interactions of integr ins with their ligands, affect a wide variety of biological functions. Here we provide evidence of how cytohesin-1, an integrin-binding protein and gu anine-nucleotide exchange factor (GEF) for ARF GTPases, regulates cell adhe sion. Mutational analyses of the beta-2 cytoplasmic domain revealed that th e adhesive function of LFA-1 depends on its interaction with cytohesin-1, u nless the integrin is activated by exogenous divalent cations. Secondly, cy tohesin-1 induces expression of an extracellular activation epitope of LFA- 1, and the exchange factor function is not essential for this activity. In contrast, LFA-1-mediated cell adhesion and spreading on inter-cellular cell adhesion molecule 1 is strongly inhibited by a cytohesin-1 mutant, which f ails to catalyze ARF GDP-GTP exchange in vitro. Thus, cytohesin-l is involv ed in the activation of LFA-1, most probably through direct interaction wit h the integrin, and induces cell spreading by its ARF-GEF activity. We ther efore propose that both direct regulation of the integrin and concomitant c hanges in the membrane topology of adherent T cells are modulated by dissec table functions of cytohesin-1.