Phenotypic screening of mutations in Pmr1, the yeast secretory pathway Ca2+/Mn2+-ATPase, reveals residues critical for ion selectivity and transport

Thirty-five mutations were generated in the yeast secretory pathway/Golgi ion pump, Pmr1, targeting oxygen-containing side chains within the predicted transmembrane segments M4, M5, M6, M7, and M8, likely to be involved in coordination of Ca2+ and Mn2+ ions. Mutants were expressed in low copy number in a yeast strain devoid of endogenous Ca2+ pumps and screened for loss of Ca2+ and Mn2+ transport on the basis of hypersensitivity to 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraaceetic acid (BAPTA) and Mn2+ toxicity, respectively. Three classes of mutants were found: mutants indistinguishable from wild type (Class 1), mutants indistinguishable from the pmr1 null strain (Class 2), and mutants with differential sensitivity to BAPTA and Mn2+ toxicity (Class 3), We show that Class 1 mutants retain normal/near normal properties, including Ca-45 transport, Golgi localization, and polypeptide conformation. In contrast, Class 2 mutants lacked any detectable Ca-45 transport; of these, a subset also showed defects in trafficking and protein folding, indicative of structural problems. Two residues identified as Class 2 mutants in this screen, Asn(774) and Asp(778) in MG, also play critical roles in related ion pumps and are therefore likely to be common architectural components of the cation-binding site. Class 3 mutants appear to have altered selectivity for Ca2+ and Mn2+ ions, as exemplified by mutant Q783A in MG. These results demonstrate the utility of phenotypic screening in the identification of residues critical for ion transport and selectivity in cation pumps.