A point mutation to Gα_i selectively blocks GoLoco motif binding: Direct evidence for Gα·GoLoco complexes in mitotic spindle dynamics

Heterotrimeric G-protein Gα subunits and GoLoco motif proteins are key members of a conserved set of regulatory proteins that influence invertebrate asymmetric cell division and vertebrate neuroepithelium and epithelial progenitor differentiation. GoLoco motif proteins bind selectively to the inhibitory subclass (Gα_i) of Gα subunits, and thus it is assumed that a Gα_i·GoLoco motif protein complex plays a direct functional role in microtubule dynamics underlying spindle orientation and metaphase chromosomal segregation during cell division. To address this hypothesis directly, we rationally identified a point mutation to Gα_i subunits that renders a selective loss-of-function for GoLoco motif binding, namely an asparagine-to-isoleucine substitution in the αD-αE loop of the Gα helical domain. This GoLoco-insensitivity ("GLi") mutation prevented Gα_i1 association with all human GoLoco motif proteins and abrogated interaction between the Caenorhabditis elegans Gα subunit GOA-1 and the GPR-1 GoLoco motif. In contrast, the GLi mutation did not perturb any other biochemical or signaling properties of Gα_i subunits, including nucleotide binding, intrinsic and RGS protein-accelerated GTP hydrolysis, and interactions with Gβγ dimers, adenylyl cyclase, and seven transmembrane-domain receptors. GoLoco insensitivity rendered Gα_i subunits unable to recruit GoLoco motif proteins such as GPSM2/LGN and GPSM3 to the plasma membrane, and abrogated the exaggerated mitotic spindle rocking normally seen upon ectopic expression of wild type Gα_i subunits in kidney epithelial cells. This GLi mutation should prove valuable in establishing the physiological roles of Gα_i·GoLoco motif protein complexes in microtubule dynamics and spindle function during cell division as well as to delineate potential roles for GoLoco motifs in receptor-mediated signal transduction.