Cofilin is an actin-depolymerizing factor, and this activity is inhibited when the protein is phosphorylated by LIM-kinase or TES-kinase. In an attempt to identify the mechanism by which the muscarinic acetylcholine receptor activated phospholipase D (PLD) downstream of the Rho effector Rho-kinase, Han et al. found that in the fibroblast cell line HEK293 and the neuroblastoma cell line N1E-115, PLD1 was not directly phosphorylated by Rho-kinase, but instead carbachol-stimulated PLD1 activation was inhibited by expression of a kinase-deficient LIM-kinase 1, which is a target for Rho-kinase. However, PLD1 was not a substrate for LIM-kinase 1 either, suggesting another step in the path from the receptor to PLD1. As cofilin is a known substrate for LIM-kinase 1, the authors overexpressed cofilin or a nonphosphorylatable mutant S3A cofilin and showed that carbachol-mediated stimulation of PLD1 was enhanced in the presence of excess wild-type cofilin and inhibited by S3A cofilin. RNAi-mediated depletion of cofilin also reduced carbachol-mediated activation of PLD1, consistent with a role for endogenous cofilin. Expression of a phosphorylation-mimic mutant of cofilin S3D also enhanced carbachol-stimulated PLD1 activation, whereas overexpression of the cofilin phosphatase slingshot inhibited PLD activation. Pull-down assays, as well as colocalization assays and coimmunoprecipitation assays with transfected cells, showed a direct interaction between PLD1 and phosphorylated, but not unphosphorylated, cofilin. If the interaction between cofilin and PLD1 was disrupted by expressing a fragment of PLD1 (F-3 fragment) responsible for mediating the interaction, then carbachol-mediated PLD1 activation was inhibited. Carbachol stimulation of stress fiber formation was also inhibited by the F-3 fragment. Carbachol stimulated the transient phosphorylation of cofilin and increased the interaction between the cofilin and PLD1 in coimmunoprecipitation assays. Phosphorylated cofilin stimulated PLD1 activity in vitro. Thus, phosphorylated cofilin is not an inactive entity but rather interacts with partners other than actin, such as PLD1.
L. Han, M. B. Stope, M. López de Jesús, P. A. Oude Weernink, M. Urban, T. Wieland, D. Rosskopf, K. Mizuno, K. H. Jakobs, M. Schmidt, Direct stimulation of receptor-controlled phospholipase D1 by phospho-cofilin. EMBO J. 26, 4189-4202 (2007). [PubMed]