LET-99 opposes Gα/GPR signaling to generate asymmetry for spindle positioning in response to PAR and MES-1/SRC-1 signaling

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Abstract
G-protein signaling plays important roles in asymmetric cell division. In C. elegans embryos, homologs of receptor-independent G protein activators, GPR-1 and GPR-2 (GPR-1/2), function together with Gα (GOA-1 and GPA-16) to generate asymmetric spindle pole elongation during divisions in the P lineage. Although Gα is uniformly localized at the cell cortex, the cortical localization of GPR-1/2 is asymmetric in dividing P cells. In this report, we show that the asymmetry of GPR-1/2 localization depends on PAR-3 and its downstream intermediate LET-99. Furthermore, in addition to its involvement in spindle elongation, Gα is required for the intrinsically programmed nuclear rotation event that orients the spindle in the one-cell. LET-99 functions antagonistically to the Gα/GPR-1/2 signaling pathway, providing an explanation for how Gα-dependent force is regulated asymmetrically by PAR polarity cues during both nuclear rotation and anaphase spindle elongation. In addition, Gα and LET-99 are required for spindle orientation during the extrinsically polarized division of EMS cells. In this cell, both GPR-1/2 and LET-99 are asymmetrically localized in response to the MES-1/SRC-1 signaling pathway. Their localization patterns at the EMS/P2 cell boundary are complementary, suggesting that LET-99 and Gα/GPR-1/2 signaling function in opposite ways during this cell division as well. These results provide insight into how polarity cues are transmitted into specific spindle positions in both extrinsic and intrinsic pathways of asymmetric cell division.