Inhibition of RhoA translocation and calcium sensitization by in vivo ADP-ribosylation with the chimeric toxin DC3B

Pretreatment of intact rabbit portal vein smooth muscle with the chimeric toxin DC3B (10^-6 M, 48 h; Aullo et al., 1993; Boquet et al. 1995) ADP-ribosylated endogenous RhoA, including cytosolic RhoA complexed with rhoGDI, and inhibited the tonic phase of phenylephrine-induced contraction and the Ca²⁺-sensitization of force by phenylephrine, endothelin and guanosine triphosphate (GTP)γS, but did not inhibit Ca²⁺-sensitization by phorbol dibutyrate. DC3B also inhibited GTPγS-induced translocation of cytosolic RhoA (Gong et al., 1997a) to the membrane fraction. In DC3B- treated muscles the small fraction of membrane-associated RhoA could be immunoprecipitated, even after exposure to GTPγS, which prevents immunoprecipitation of non-ADP-ribosylated RhoA. Dissociation of cytosolic RhoA-rhoGDI complexes with SDS restored the immunoprecipitability and ADP ribosylatability of RhoA, indicating that both the ADP-ribosylation site (Asn 41) and RhoA insert loop (Wei et al., 1997) are masked by rhoGDI and that the long axes of the two proteins are in parallel in the heterodimer. We conclude that RhoA plays a significant role in G-protein-, but not protein kinase C- mediated, Ca²⁺ sensitization and that ADP ribosylation inhibits in vivo the Ca²⁺-sensitizing effect of RhoA by interfering with its binding to a membrane-associated effector.