A Point Mutation to Gαi Selectively Blocks GoLoco Motif Binding: DIRECT EVIDENCE FOR Gα·GoLoco COMPLEXES IN MITOTIC SPINDLE DYNAMICS*S⃞

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 (Gαi) of Gα subunits, and thus it is assumed that a Gαi·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 Gαi 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 Gαi1 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 Gαi 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 Gαi 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 Gαi subunits in kidney epithelial cells. This GLi mutation should prove valuable in establishing the physiological roles of Gαi·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.