Gβγ regulates mitotic Golgi fragmentation and G2/M cell cycle progression

Heterotrimeric G proteins (alpha beta gamma) function at the cytoplasmic surface of a cell's plasma membrane to transduce extracellular signals into cellular responses. However, numerous studies indicate that G proteins also play noncanonical roles at unique intracellular locations. Previous work has established that G protein beta gamma subunits (alpha beta gamma) regulate a signaling pathway on the cytoplasmic surface of Golgi membranes that controls the exit of select protein cargo. Now, we demonstrate a novel role for alpha beta gamma in regulating mitotic Golgi fragmentation, a key checkpoint of the cell cycle that occurs in the late G2 phase. We show that small interfering RNA-mediated depletion of G beta 1 and G beta 2 in synchronized cells causes a decrease in the number of cells with fragmented Golgi in late G2 and a delay of entry into mitosis and progression through G2/M. We also demonstrate that during G2/M alpha beta gamma acts upstream of protein kinase D and regulates the phosphorylation of the Golgi structural protein GRASP55. Expression of Golgi-targeted GRK2ct, a alpha beta gamma-sequestering protein used to inhibit alpha beta gamma signaling, also causes a decrease in Golgi fragmentation and a delay in mitotic progression. These results highlight a novel role for alpha beta gamma in regulation of Golgi structure.