Regulation by GDI of RhoA/Rho-kinase-induced Ca sensitization of smooth muscle myosin II

Gong, Ming Cui, Isabelle Gorenne, Paul Read, Taiping Jia, Robert K. Nakamoto, Avril V. Somlyo, and Andrew P. Somlyo. Regulation by GDI of RhoA/Rho-kinase-induced Ca21 sensitization of smooth muscle myosin II. Am J Physiol Cell Physiol 281: C257–C269, 2001.—We characterized the role of guanine nucleotide dissociation inhibitor (GDI) in RhoA/Rho-kinase-mediated Ca21 sensitization of smooth muscle. Endogenous contents (;2–4 mM) of RhoA and RhoGDI were near stoichiometric, whereas a supraphysiological GDI concentration was required to relax Ca21 sensitization of force by GTP and guanosine 59-O-(3-thiotriphosphate) (GTPgS). GDI also inhibited Ca21 sensitization by GTP zG14V RhoA, by a-adrenergic and muscarinic agonists, and extracted RhoA from membranes. GTPgS translocated Rho-kinase to a Triton X-114-extractable membrane fraction. GTP zG14V RhoA complexed with GDI also induced Ca21 sensitization, probably through in vivo dissociation of GTP zRhoA from the complex, because it was reversed by addition of excess GDI. GDI did not inhibit Ca21 sensitization by phorbol ester. Constitutively active Cdc42 and Rac1 inhibited Ca21 sensitization by GTP zG14V RhoA. We conclude that 1) the most likely in vivo function of GDI is to prevent perpetual “recycling” of GDP zRhoA to GTP zRhoA; 2) nucleotide exchange (GTP for GDP) on complexed GDP z RhoA/GDI can precede translocation of RhoA to the membrane; 3) activation of Rho-kinase exposes a hydrophobic domain; and 4) Cdc42 and Rac1 can inhibit Ca21 sensitization by activated GTP zRhoA.