MEX-5 and MEX-6 function to establish soma/germline asymmetry in early C. elegans embryos.

An asymmetrical network of cortically localized PAR proteins forms shortly after fertilization of the C. elegans egg. This network is required for subsequent asymmetries in the expression patterns of several proteins that are encoded by nonlocalized, maternally expressed mRNAs. We provide evidence that two nearly identical genes, mex-5 and mex-6, link PAR asymmetry to those subsequent protein asymmetries. MEX-5 is a novel, cytoplasmic protein that is localized through PAR activities to the anterior pole of the 1-cell stage embryo. MEX-5 localization is reciprocal to that of a group of posterior-localized proteins called germline proteins. Ectopic expression of MEX-5 is sufficient to inhibit the expression of germline proteins, suggesting that MEX-5 functions to inhibit anterior expression of the germline proteins.