The non-core (regulatory) subunits of eukaryotic translation initiation factor 3 (eIF3) from the fission yeast Schizosaccharomyces pombe

Abstract

The fission yeast Schizosaccharomyces pombe homologue of the p40/eIF3h subunit of mammalian translation initiation factor eIF3 has been characterized in this study. We show that eIF3h physically associates with the 40S ribosomal particles as a constituent of the multimeric eIF3 protein complex that consists of all five known eIF3 core subunits, eIF3a, eIF3b, eIF3c, eIF3g and eIF3i as well as the five non-core subunits eIF3d, eIF3e, eIF3f, eIF3h, and eIF3m that constitute an eIF3 holocomplex in fission yeast. However, affinity purification of eIF3 from fission yeast cells expressing TAP-tagged eIF3h suggests the presence of distinct forms of eIF3 that differ in their composition of the non-core subunits. Further characterization of eIF3h shows that strains lacking eij3h + (eif3hDelta) are viable and show no gross defects either in vegetative growth or in the rate of in vivo protein synthesis. Polysome profile analysis shows no apparent defects in translation initiation. Furthermore, deletion of eif3h+ does not affect the ability of the other eIF3 subunits to remain associated with one another in a tight protein complex similar to the situation in wild-type cells. Interestingly, mutant eif3hDelta cells show several prominent phenotypic properties. They are hypersensitive to caffeine and highly defective in conjugation/meiosis producing either no spores or incomplete tetrads with a very high frequency. Consistent with the presence of an MPN (Mpr1-Pad1- N-terminal) domain in both eIF3h and the proteasomal lid subunit Rpn11, we observed that eif3h+ genetically interacts with rpn11+ in S. pombe . Furthermore, we observe that eIF3h physically associates with the proteasomal machinery. eif3hDelta cells are also hypersensitive to drugs that inhibit the growth of fission yeast mutants defective in ubiquitin-dependent proteolysis. Since the proteasome is known to be involved in meiosis, our observation that eIF3h genetically interacts with the proteasome and is also required for meiotic division hints at the possibility that this specific interaction may play a role in meiotic division leading to spore formation in S. pombe. Surprisingly, we observed that the gene eif3f+, encoding another eIF3 non-core subunit eIF3f, is essential for viability of S. pombe. The implications of these findings in relation to the function of the non-core subunits of eIF3 are discussed.