Syndesmos, a Syndecan-4 Cytoplasmic Domain Interactor, Binds to the Focal Adhesion Adaptor Proteins Paxillin and Hic-5

Abstract Syndecan-4 and integrins are the primary transmembrane receptors of focal adhesions in cells adherent to extracellular matrix molecules. Syndesmos is a cytoplasmic protein that interacts specifically with the cytoplasmic domain of syndecan-4, and it co-localizes with syndecan-4 in focal contacts. In the present study we sought possible interactors with syndesmos. We find that syndesmos interacts with the focal adhesion adaptor protein paxillin. The binding of syndesmos to paxillin is direct, and these interactions are triggered by the activation of protein kinase C. Syndesmos also binds the paxillin homolog, Hic-5. The connection of syndecan-4 with paxillin through syndesmos parallels the connection between paxillin and integrins and may thus reflect the cooperative signaling of these two receptors in the assembly of focal adhesions and actin stress fibers.