Optogenetic dissection of mitotic spindle positioning in vivo
The position of the mitotic spindle determines the plane of cell cleavage, and thereby the location, size, and content of daughter cells. Spindle positioning is driven by dynein-mediated pulling forces exerted on astral microtubules. This process requires an evolutionarily conserved complex of G-GDP, GPR-1/2Pins/LGN, and LIN-5Mud/NuMA proteins. It remains unknown whether this complex merely forms a membrane anchor for dynein, or whether the individual components have additional functions, for instance through G-GTP or dynein activation. To functionally dissect this system, we developed a genetic strategy for light-controlled localization of endogenous proteins in C. elegans embryos. Controlled germline expression and membrane recruitment of the G regulators RIC-8Ric-8A and RGS-7Loco/RGS3, and replacement of G with a light-inducible membrane anchor demonstrated that G-GTP signaling is dispensable for pulling force generation. In the absence of G, cortical recruitment of GPR-1/2 or LIN-5, but not dynein itself, induced high pulling forces. Local recruitment of LIN-5 overruled normal cell-cycle and polarity regulation, and provided experimental control over the spindle and cell cleavage plane. Our results define G*GDP-GPR-1/2Pins/LGN as a regulatable membrane anchor, and LIN-5Mud/NuMA as a potent activator of dynein-dependent spindle positioning forces. This study also highlights the possibilities for optogenetic control of endogenous proteins within an animal system.