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J. Biol. Chem., Vol. 275, Issue 29, 22229-22237, July 21, 2000

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Regulation of F-actin and Endoplasmic Reticulum Organization by the Trimeric G-protein G_i2 in Rat Hepatocytes
IMPLICATION FOR THE ACTIVATION OF STORE-OPERATED Ca²⁺ INFLOW^*

Ying-Jie Wang, Roland B. Gregory, and Greg J. Barritt^§

From the Department of Medical Biochemistry, School of Medicine, Faculty of Health Sciences, Flinders University, GPO Box 2100, Adelaide, South Australia 5001, Australia

The roles of the heterotrimeric G-protein, G_i2, in regulating the actin cytoskeleton and the activation of store-operated Ca²⁺ channels in rat hepatocytes were investigated. G_i2 was principally associated with the plasma membrane and microsomes. Both F-actin and G_i2 were detected by Western blot analysis in a purified plasma membrane preparation, the supernatant and pellet obtained by treating the plasma membrane with Triton X-100, and after depolymerization and repolymerization of F-actin in the Triton X-100-insoluble pellet. Actin in the Triton X-100-soluble supernatant co-precipitated with G_i2 using either anti-G_i2 or anti-actin antibodies. The principally cortical location of F-actin in hepatocytes cultured for 0.5 h changed to a pericanalicular distribution over a further 3.5 h. Some G_i2 co-localized with F-actin at the plasma membrane. Pretreatment with pertussis toxin ADP-ribosylated 70-80% of G_i2 in the plasma membrane and microsomes, prevented the redistribution of F-actin, caused redistribution and fragmentation of the endoplasmic reticulum, and inhibited vasopressin-stimulated Ca²⁺ inflow. It is concluded that (i) a significant portion of hepatocyte G_i2 associates with, and regulates the arrangement of, cortical F-actin and the endoplasmic reticulum and (ii) either or both of these regulatory roles are likely to be required for normal vasopressin activation of Ca²⁺ inflow.

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