The Rho kinases (ROCK I and II) regulate different aspects of myosin II‐driven contractility

The small GTPase Rho and its downstream Rho kinases (ROCK I and II) are implicated in microfilament bundle assembly and smooth muscle contractility. ROCK I and II were previously assumed to be functionally redundant but we show that endogenous ROCKs are distinctly regulated and are associated with distinct myosin compartments (1, 2). Depletion of ROCK I by siRNA led to complete abrogation of stress fibers and focal adhesions, even though ROCK II remained. However, ROCK II‐depleted cells had impaired phagocytosis of fibronectin‐coated beads and matrix assembly. Total internal reflection fluorescence microscopic analysis of myosin light chain‐GFP expressing fibroblasts revealed that a submembranous pool of myosin light chain in control cells was missing in ROCK II‐deficient cells and replaced by ROCK I‐dependent stress fibers. Overexpression of a constitutively active form of myosin light chain reconstituted the submembranous pool of myosin II and the fibronectin matrix assembly defect. These effects were related to partly distinct lipid‐binding preferences of pleckstrin homology domains present in each ROCK. ROCK II bound phosphatidylinositol 3,4,5P 3 and was sensitive to its levels, properties not shared by ROCK I. Therefore, homologous mammalian rho kinases are not redundant in fibroblasts although they can phosphorylate the same substrates in vitro.