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Abstract

Heterotrimeric G proteins promote microtubule forces that position mitotic spindles during asymmetric cell division in C. elegans embryos. While all previously studied G protein functions require activation by seven-transmembrane receptors, this function appears to be receptor independent. We found that mutating a regulator of G protein signaling, RGS-7, resulted in hyperasymmetric spindle movements due to decreased force on one spindle pole. RGS-7 is localized at the cell cortex, and its effects require two redundant Galpha(o)-related G proteins and their nonreceptor activators RIC-8 and GPR-1/2. Using recombinant proteins, we found that RIC-8 stimulates GTP binding by Galpha(o) and that the RGS domain of RGS-7 stimulates GTP hydrolysis by Galpha(o), demonstrating that Galpha(o) passes through the GTP bound state during its activity cycle. While GTPase activators typically inactivate G proteins, RGS-7 instead appears to promote G protein function asymmetrically in the cell, perhaps acting as a G protein effector.