Localization of a heterotrimeric G protein gamma subunit to focal adhesions and associated stress fibers

Signal transducing heterotrimeric G proteins are responsible for coupling a large number of cell surface receptors to the appropriate effector(s). Of the three subunits, 16 alpha, 4 beta, and 5 gamma subunits have been characterized, indicating a potential for over 300 unique combinations of heterotrimeric G proteins. To begin deciphering the unique G protein combinations that couple specific receptors with effectors, we examined the subcellular localization of the gamma subunits. Using anti-peptide antibodies specific for each of the known gamma subunits, neonatal cardiac fibroblasts were screened by standard immunocytochemistry. The anti-gamma 5 subunit antibody yielded a highly distinctive pattern of intensely fluorescent regions near the periphery of the cell that tended to protrude into the cell in a fibrous pattern. Dual staining with anti-vinculin antibody showed co-localization of the gamma 5 subunit with vinculin. In addition, the gamma 5 subunit staining extended a short distance out from the vinculin pattern along the protruding stress fiber, as revealed by double staining with phalloidin. These data indicated that the gamma 5 subunit was localized to areas of focal adhesion. Dual staining of rat aortic smooth muscle cells and Schwann cells also indicated co-localization of the gamma 5 subunit and vinculin, suggesting that the association of the gamma 5 subunit with areas of focal adhesion was wide-spread.