Sum1, a component of the fission yeast eIF3 translation initiation complex, is rapidly relocalized during environmental stress and interacts with components of the 26S proteasome.

Eukaryotic translation initiation factor 3 (eIF3) is a multisubunit complex that plays a central role in translation initiation. We show that fission yeast Sum1, which is structurally related to known eIF3 subunits in other species, is essential for translation initiation, whereas its overexpression results in reduced global translation. Sum1 is associated with the 40S ribosome and interacts stably with Int6, an eIF3 component, in vivo, suggesting that Sum1 is a component of the eIF3 complex. Sum1 is cytoplasmic under normal growth conditions. Surprisingly, Sum1 is rapidly relocalized to cytoplasmic foci after osmotic and thermal stress. Int6 and p116, another putative eIF3 subunit, behave similarly, suggesting that eIF3 is a dynamic complex. These cytoplasmic foci, which additionally comprise eIF4E and RNA components, may function as translation centers during environmental stress. After heat shock, Sum1 additionally colocalizes stably with the 26S proteasome at the nuclear periphery. The relationship between Sum1 and the 26S proteasome was further investigated, and we find cytoplasmic Sum1 localization to be dependent on the 26S proteasome. Furthermore, Sum1 interacts with the Mts2 and Mts4 components of the 26S proteasome. These data indicate a functional link between components of the structurally related eIF3 translation initiation and 26S proteasome complexes.