Zinc status and vacuolar zinc transporters control alkaline phosphatase accumulation and activity in Saccharomyces cerevisiae.

Little is known about how metalloproteins in the secretory pathway obtain their metal ion cofactors. We used the Pho8 alkaline phosphatase of the yeast Saccharomyces cerevisiae to probe this process in vivo. We found that both Pho8 activity and protein accumulation are zinc-dependent and decrease in zinc-limited cells. Low Pho8 accumulation was the result of degradation by vacuolar proteases. Surprisingly, the protective effect of zinc on Pho8 stability was not solely due to Zn2+ binding to the active-site ligands suggesting that the Pho8 protein is targeted for degradation in zinc-limited cells by another mechanism. Pho8 appears to be a rare example of a metalloprotein whose stability is regulated by its metal cofactor independently of active-site binding. We also assessed which zinc transporters are responsible for supplying zinc to Pho8. We found that the Zrc1 and Cot1 vacuolar zinc transporters play the major role while the Msc2/Zrg17 zinc transporter complex active in the endoplasmic reticulum is not involved. These results demonstrate that the vacuolar zinc transporters, previously implicated in metal detoxification, also deliver zinc to certain metalloproteins within intracellular compartments. These data suggest that Pho8 receives its metal cofactor in the vacuole rather than in earlier compartments of the secretory pathway.