Establishment and maintenance of cell polarity are essential for diversity of many animal cells. In the C. elegans embryo the conserved PAR proteins control the anterior cytoplasmic localization of two redundant CCCH tandem zinc finger proteins, MEX-5 and MEX-6. MEX-5/6 specify somatic and germ cell fates, however how they achieve this is not completely understood. To find partners of MEX-5 we performed MEX-5 immunoprecipitation followed by mass spectrometry. Consistent with yeast data were CCCH proteins have a role in mRNA decay we found MEX-5 in a complex with mRNA degradation proteins. In
mex-5 and
mex-5;
mex-6 mutant embryos, the enrichment of two Class II mRNA,
pos-1 and
mex-1, are lost in the germline precursors and the mRNA levels remain high in all cells [1 and our results]. By northern blot, we show that the total levels of class II mRNAs are increased in
mex-5 and
mex-5;
mex-6 mutants. This suggests that MEX-5 could regulate the degradation of class II mRNAs in somatic blastomeres. We also found that MEX-5 immunoprecipitates with RNA polII and several members of the Mediator complex. We could not identify a role for MEX-5 in transcription in early embryos. However, as the yeast and mammalian homologues, we found that MEX-5 is shuttling in the nuclei of somatic cells. Nuclear shuttling is inhibited in the P lineage by PAR-1 phosphorylation. Moreover, the shuttling is dependent on MEX-5 ability to bind mRNA as a MEX-5 CAAH mutant that cannot bind mRNA is retained in the nucleus. We propose that in somatic cells, MEX-5 binds target mRNAs in the nucleus, is then exported in the cytoplasm where the mRNA can be degraded. In the P lineage MEX-5 is kept inactive by PAR-1 phosphorylation allowing the maintenance of germline specific mRNAs. 1. Tenlen JR et al (2006). Genetics 174(4): 1933-45.