Extracellular control of PAR protein localization during asymmetric cell division in the C. elegans embryo.

The axis of asymmetric cell division is controlled to determine the future position of differentiated cells during animal development. The asymmetric localization of PAR proteins in the Drosophila neuroblast and C. elegans embryo are aligned with the axes of the embryo. However, whether extracellular or intracellular signals determine the orientation of the localization of PAR proteins remains controversial. In C. elegans, the P0 zygote and germline cells (P1, P2, and P3) undergo a series of asymmetric cell divisions. Interestingly, the axis of the P0 and P1 divisions is opposite to that of the P2 and P3 divisions. PAR-2, a ring-finger protein, and PAR-1, a kinase, relocalize to the anterior side of the P2 and P3 germline precursors at the site of contact with endodermal precursors. Using an in vitro method, we have found that the PAR-2 protein is distributed asymmetrically in the absence of extracellular signals, but the orientation of the protein localization in the P2 and P3 cells is determined by contact with endodermal precursor cells. Our mutant analyses suggest that mes-1 and src-1, which respectively encode a transmembrane protein and a tyrosine kinase, were not required to establish the asymmetric distribution of PAR-2, but were required to determine its orientation at the site of contact with the endodermal precursors. The PAR-2 localization during the asymmetric P2 and P3 divisions is controlled by extracellular signals via MES-1/SRC-1 signaling. Our findings suggest that Src functions as an evolutionarily conserved molecular link that coordinates extrinsic cues with PAR protein localization.