Dynamic localization of LIN-5 and GPR-1/2 to cortical force generation domains during spindle positioning.

G protein signaling pathways regulate mitotic spindle positioning during cell division in many systems. In Caenorhabditis elegans embryos, G alpha subunits act with the positive regulators GPR-1/2 and LIN-5 to generate cortical pulling forces for posterior spindle displacement during the first asymmetric division. GPR-1/2 are asymmetrically localized at the posterior cortex by PAR polarity cues at this time. Here we show that LIN-5 colocalizes with GPR-1/2 in one-cell embryos during spindle displacement. Significantly, we also find that LIN-5 and GPR-1/2 are localized to the opposite, anterior cortex in a polarity-dependent manner during the nuclear centration and rotation movements that orient the forming spindle onto the polarity axis. The depletion of LIN-5 or GPR-1/2 results in decreased centration and rotation rates, indicating a role in force generation at this stage. The localization of LIN-5 and GPR-1/2 is largely interdependent and requires G alpha. Further, LIN-5 immunoprecipitates with G alpha in vivo, and this association is GPR-1/2 dependent. These results suggest that a complex of G alpha/GPR-1/2/LIN-5 is asymmetrically localized in response to polarity cues, and this may be the active signaling complex that transmits asymmetries to the force generation machinery during both nuclear rotation and spindle displacement.

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