Manganese selectivity of Pmr1, the yeast secretory pathway ion pump, is defined by residue Gln⁷⁸³ in transmembrane segment 6: Residue Asp⁷⁷⁸ is essential for cation transport

We have solubilized and purified the histidine-tagged yeast secretory pathway/Golgi ion pump Pmr1 to near homogeneity in one step, using nickel affinity chromatography. The purified pump demonstrates both Ca²⁺-and Mn²⁺-dependent ATP hydrolysis and phosphoenzyme intermediate formation in forward (ATP) and reverse (P(i)) directions. This preparation has allowed us to examine, in detail, the properties of mutations D778A and Q783A in transmembrane segment M6 of Pmr1. In phenotypic screens of Ca²⁺ chelator and Mn²⁺ toxicity reported separately (Wei, Y., Chen, J., Rosas, G., Tompkins, D.A., Holt, P.A., and Rao, R. (2000) J. Biol. Chem. 275, XXXX-XXXX), D778A was a loss-of-function mutant apparently defective for transport of both Ca²⁺ and Mn²⁺, whereas mutant Q783A displayed a differential sensitivity consistent with the selective loss of Mn²⁺ transport. We show that mutant D778A is devoid of cation-dependent ATP hydrolytic activity and phosphoenzyme formation from ATP. However, reverse phosphorylation from P(i) is preserved but is insensitive to inhibition by Ca²⁺ or Mn²⁺ ions, which is evidence for a specific inability to bind cations in this mutant. We also show that Ca²⁺ can activate ATP hydrolysis in the purified Q783A mutant, with a half-maximal concentration of 0.06 μM, essentially identical to that of wild type (0.07 μM). Mn²⁺ activation of ATP hydrolysis was half-maximal at 0.02 μM in wild type, establishing a normal selectivity profile of Mn²⁺ > Ca²⁺. Strikingly, Mn²⁺-ATPase in the Q783A mutant was nearly abolished, even at concentrations of up to 10 μM. These results were confirmed in assays of phosphoenzyme intermediates. Molecular modeling of the packing between helices M4 and M6 suggests that residue Gln⁷⁸³ in M6 may form a critical hydrophobic interaction with Val³³⁵ in M4, such that the Ala substitution modifies the packing or tilt of the helices and thus the ion pore. The data emphasize the critical role of transmembrane segment M6 in defining the cation binding pocket of P-type ATPases.