OOC-3, a novel putative transmembrane protein required for establishment of cortical domains and spindle orientation in the P-1 blastomere of C. elegans embryos

Asymmetric cell divisions require the establishment of an axis of polarity, which is subsequently communicated to downstream events, During the asymme tric cell division of the P-1 blastomere in C. elegans, establishment of po larity depends on the establishment of anterior and posterior cortical doma ins, defined by the localization of the PAR proteins, followed by the orien tation of the mitotic spindle along the previously established axis of pola rity, To identify genes required for these events, we have screened a colle ction of maternal-effect lethal mutations on chromosome II of C, elegans, W e have identified a mutation in one gene, ooc-3, with mis-oriented division axes at the two-cell stage. Here we describe the phenotypic and molecular characterization of ooc-3, ooc-3 is required for the correct localization o f PAR-2 and PAR-3 cortical domains after the first cell division, OOC-3 is a novel putative transmembrane protein, which localizes to a reticular memb rane compartment, probably the endoplasmic reticulum, that spans the whole cytoplasm and is enriched on the nuclear envelope and cell-cell boundaries. Our results show that ooc-3 is required to form the cortical domains essen tial for polarity after cell division.