Jean-Claude Labb1, Anne Pacquelet, Esther Zanin, Thomas Marty and Monica Gotta. Asymmetric cell division is a fundamental process for the generation of cell diversity during the development of metazoans. We are interested in understanding how cells become polarized and divide asymmetrically. In the early C. elegans embryo PAR proteins are responsible for the establishment and maintenance of cell polarity. PAR-3, PAR-6 and PKC-3 are members of a protein complex localized at the anterior cortex of the embryo (referred to as the PAR-3/6/3 complex); the PAR-2 and PAR-1 proteins are present at the posterior cortex of the embryo. Mutations that disrupt PAR-2 function result in embryos in which the PAR-3/6/3 complex is mislocalized, leading to polarity defects and embryonic lethality. It has been previously shown that the lethality of
par-2 mutants can be suppressed by reducing PAR-6 levels or by depleting CDC-42. This indicates that the lethality due to the absence of PAR-2 can be suppressed by conditions that affect the levels, the localization and/or the activity of the PAR-3/6/3 complex. . To identify new genes that either regulate or are regulated by the PAR-3/6/3 complex, we have performed a genome-wide RNAi screen looking for suppressors of
par-2 lethality. One of the identified suppressors is NOS-3, the homologue of Drosophila Nanos. We find that
nos-3 suppresses most of the phenotypes associated with loss of
par-2 function, including early cell division defects, maternal effect sterility and mislocalization of the anterior PAR-3/6/3 complex. Strikingly, we find that asymmetric localization of PAR-1 at the posterior cortex is not restored. However, PAR-1 activity is essential in
nos-3;
par-2 double mutants, suggesting that the function of PAR-1 is independent of its cortical localization. Interestingly,
nos-3 can also suppress the phenotypes associated with a null allele of
par-2, indicating that NOS-3 impinges on the PAR pathway independently of the PAR-2 protein. Consistent with this, we find that NOS-3 regulates PAR-6 levels in the embryo. We are currently investigating the molecular mechanisms by which NOS-3 controls PAR-6 levels.