In Caenorhabditis elegans, the Mex-3 protein is a translational regulator that specifies the posterior blastomere identity in the early embryo and contributes to the maintenance of the germline totipotency. We have now identified a family of four homologous human Mex-3 genes, called hMex-3A to -3D that encode proteins containing two heterogeneous nuclear ribonucleoprotein K homology (KH) domains and one carboxy-terminal RING finger module. The hMex-3 are phosphoproteins that bind RNA through their KH domains and shuttle between the nucleus and the cytoplasm via the CRM1-dependent export pathway. Our analysis further revealed that hMex-3A and hMex-3B, but not hMex-3C, colocalize with both the hDcp1a decapping factor and Argonaute (Ago) proteins in processing bodies (P bodies), recently characterized as centers of mRNA turnover. Taken together, these findings indicate that hMex-3 proteins constitute a novel family of evolutionarily conserved RNA-binding proteins, differentially recruited to P bodies and potentially involved in post-transcriptional regulatory mechanisms.

The novel RNA-binding E3 ubiquitin ligase MEX-3C targets the mRNA of MHC-I gene HLA-A2 for degradation in a RING-dependent manner. Depletion of MEX-3C in NK cells leads to reduced killing of target cells.