SPN-2, an eIF4E-binding protein required for spindle positioning and oogenesis in Caenorhabditis elegans

Translational regulation plays an essential role in the spatial and temporal restriction of protein expression during development. In C. elegans , many mRNA binding proteins that play a role in translational regulation in the germ line and embryo have been identified, but the precise mechanisms by which they repress translation are still being elucidated. In particular, little is known about link between these mRNA binding proteins and the translation machinery. Here I report the identification of an eIF4E-binding protein (4E-BP) from C. elegans called SPN-2. Loss of spn-2 function results in defects in spindle positioning and other microtubule dependent processes in the early embryo. In addition, spn-2 worms become sterile at restrictive temperature and display defects in oogenesis. SPN-2 shares limited sequence homology with 4E-BPs from other species and can associate with four of the five C. elegans eIF4E isoforms. Consistent with a role in translation, SPN-2 localizes to the cytoplasm and germ-line P granules, ribonucleoprotein complexes thought to be involved in translational regulation. SPN-2 is also detected in small somatic granules in late embryos. Normal localization of SPN-2 to the P granules requires the germ granule specific protein PGL-1. I showed defects in spindle positioning and other microtubule dependent processes in spn-2 embryos were due in large part to the ectopic expression of the microtubule-severing protein, MEI-1. I also showed SPN-2 functions in parallel to MEL-26 mediated cullin degradation pathway of MEI-1. Further, SPN-2 binds to the RNA-binding protein OMA-1, which in turn binds to the mei-1 3'UTR. These results suggest SPN-2 could repress the translation of mei-1 in mitotic embryos and it could be recruited to mei-1 mRNA by OMA-1. Further evidence for a role of SPN-2 in translational repression comes from the ectopic expression of a nanos 3 'UTR reporter gene in spn-2 mutant oocytes. Phenotypic analysis and quantification of other protein levels support the view that SPN-2 is not a general repressor of translation, but rather acts on a subset of messages. Cumulatively, these results suggest that SPN-2 functions as a 4E-BP to negatively regulate mRNAs during oogenesis and the early mitotic divisions of the embryo.