Plectin-RACK1 (Receptor for Activated C Kinase 1) Scaffolding

Abstract

Agonist-induced translocation of protein kinase C (PKC) isozymes is mediated by receptors for the activated form of the kinase, shuttling it from one intracel-lular site to another and enhancing its catalytic activity. It is however unknown whether the receptors themselves are anchored to certain intracellular structures prior to their engagement with PKC. We show here se-questering of receptor for activated C kinase 1 (RACK1) to the cytoskeleton through the cytoskeletal linker protein plectin during the initial stages of cell adhesion. We found that upon PKC activation, RACK1 was released from the cytoskeleton and transferred to the detergent-soluble cell compartment, where it formed an inducible triple complex with one of the PKC isozymes, PKC, and with plectin. In plectin-deficient cells the cytoskeleton-associated RACK1 fraction was reduced, and the protein was found predominantly at sites to which it normally translocated upon PKC activation. Concomitantly, dis-location of PKC and elevated enzymatic activity were observed in these cells. PKC was also more rapidly degraded, likely due to its overactivation. We propose a previously unrecognized function of plectin as cytoskel-etal regulator of PKC signaling, and possibly other sig-naling events, through sequestration of the scaffolding protein RACK1.