The medial-Golgi Ion Pump Pmr1 Supplies the Yeast Secretory Pathway with Ca21 and Mn21 Required for Glycosylation, Sorting, and Endoplasmic Reticulum- Associated Protein Degradation

The yeast Ca21 adenosine triphosphatase Pmr1, located in medial-Golgi, has been implicated in intracellular transport of Ca21 and Mn21 ions. We show here that addition of Mn21 greatly alleviates defects of pmr1 mutants in N-linked and O-linked protein glycosylation. In contrast, accurate sorting of carboxypeptidase Y (CpY) to the vacuole requires a sufficient supply of intralumenal Ca21. Most remarkably, pmr1 mutants are also unable to degrade CpY*, a misfolded soluble endoplasmic reticulum protein, and display phenotypes similar to mutants defective in the stress response to malfolded endoplasmic reticulum proteins. Growth inhibition of pmr1 mutants on Ca21-deficient media is overcome by expression of other Ca21 pumps, including a SERCA-type Ca21 adenosine triphosphatase from rabbit, or by Vps10, a sorting receptor guiding non-native luminal proteins to the vacuole. Our analysis corroborates the dual function of Pmr1 in Ca21 and Mn21 transport and establishes a novel role of this secretory pathway pump in endoplasmic reticulum-associated processes.