SPIN90-IRSp53 complex participates in Rac-induced membrane ruffling.

SPIN90 is a key regulator of actin cytoskeletal organization. Using the BioGRID(beta) database (General Repository for Interaction Datasets), we identified IRSp53 as a binding partner of SPIN90, and confirmed the in vivo formation of a SPIN90-IRSp53 complex mediated through direct association of the proline-rich domain (PRD) of SPIN90 with the SH3 domain of IRSp53. SPIN90 and IRSp53 positively cooperated to mediate Rac activation, and co-expression of SPIN90 and IRSp53 in COS-7 cells led to the complex formation of SPIN90-IRSp53 in the leading edge of cells. PDGF treatment induced strong colocalization of SPIN90 and IRSp53 at membrane protrusions. Within such PDGF-induced protrusions, knockdown of SPIN90 protein using siRNA significantly reduced lamellipodia-like protrusions as well as localization of IRSp53 at those sites. Finally, competitive inhibition of SPIN90-IRSp53 binding by SPIN90 PRD dramatically reduced ruffle formation, further suggesting that SPIN90 plays a key role in the formation of the membrane protrusions associated with cell motility.