Dynamic modulation of cytoskeletal proteins linking integrins to signalingcomplexes in spreading cells - Role of skelemin in initial integrin-induced spreading

Recently we showed that signaling across beta (3)-integrin leads to activat ion of calpain and formation of integrin clusters that are involved in Rac activation. The subsequent activation of Rac and Rho leads to the formation of focal complexes and focal adhesions, respectively. The goal of the pres ent study was to determine whether different, proteins link the integrin to the cytoskeleton in the different complexes. We show that talin is present in focal adhesions but not in the calpain-induced clusters. alpha -Actinin colocalized with integrin at various sites, including the calpain-induced clusters. Skelemin, a protein shown recently to interact with beta (1)- and beta (3)-integrin in vitro, colocalized with integrin in calpain-induced c lusters but was absent from focal adhesions. Cells transiently expressing s kelemin C2 motifs, which contain the integrin binding site, failed to form integrin clusters or to spread on a substrate for beta (1)- and beta (3)-in tegrins. These results 1) suggest a dynamic reorganization of integrin comp lexes during cell spreading, 2) show that different cytoskeletal proteins l ink integrins in different complexes, and 3) demonstrate that skelemin is r esponsible for linking integrin to the calpain-induced clusters, and 4) sho w that the integrin-skelemin interaction is essential for transmission of s ignals leading to the initial steps of cell spreading.