YKE4 (YIL023C) encodes a bidirectional zinc transporter in the endoplasmic reticulum of Saccharomyces cerevisiae.

YIL023C encodes a member of the SLC39A, or ZIP, family, which we refer to as yeast KE4 (YKE4) after its mouse ortholog. Yke4p was localized to the endoplasmic reticulum (ER) membrane using Yke4p-specific antiserum. YKE4 is not an essential gene; however, deletion of YKE4 resulted in a sensitivity to calcofluor white and poor growth at 36 degrees C on respiratory substrates containing high zinc. Overexpression of transition metal transporters Zrc1p and Cot1p or the mouse orthologue mKe4 in Deltayke4 suppressed the poor growth at 36 degrees C on respiratory substrates. We found that the role of Yke4p depends on the zinc status of the cells. In a zinc-adequate environment, Yke4p transports zinc into the secretory pathway, and the deletion of YKE4 leads to a zinc-suppressible cell wall defect. In high zinc medium, transport of zinc into the secretory pathway through Yke4p is a way to eliminate zinc from the cytosol, and deletion of YKE4 leads to toxic zinc accumulation in the cytosol. Under low cytosolic zinc conditions, however, Yke4p removes zinc from the secretory pathway, and deletion of YKE4 partially compensates for the loss of Msc2p, an ER zinc importer, and therefore helps to alleviate ER stress. In our model, Yke4p balances zinc levels between the cytosol and the secretory pathway, whereas the previously described Msc2p-Zrg17p ER zinc importer complex functions mainly in zinc-depleted conditions to ensure a ready supply of zinc essential for ER functions, such as phospholipid biosynthesis and unfolded protein response.