Expression of the yeast plasma membrane [H⁺]ATPase in secretory vesicles: A new strategy for directed mutagenesis

Secretory vesicles that accumulate in the temperature-sensitive sec6-4 strain of yeast have been shown to contain a vanadate-sensitive ATPase, presumably en route to the plasma membrane (Walworth, N. C., and Novick, P. J. (1987) J. Cell Biol. 105, 163-174). We have now established this enzyme to be a fully functional form of the PMA1 [H⁺]ATPase, identical in its catalytic properties to that found in the plasma membrane. In addition, the secretory vesicles are sealed tightly enough to permit the measurement of ATP-dependent proton pumping with fluorescent probes. We have gone on to exploit the vesicles as an expression system for site-directed mutants of the ATPase. For this purpose, a sec6-4 strain has been constructed in which the chromosomal PMA1 gene is under control of the GAL1 promoter; the mutant pma1 allele to be studied is introduced on a centromeric plasmid under the control of a novel heat shock promoter. In galactose medium at 23°C, the wild-type ATPase is produced and supports normal vegetative growth. When the cells are switched to glucose medium at 37°C, however, the wild-type gene turns off, the mutant gene turns on, and secretory vesicles accumulate. The vesicles contain a substantial amount of newly synthesized, plasmid-encoded ATPase (5-10% of total vesicle protein), but only traces of residual wild-type PMA1 ATPase and no detectable mitochondrial ATPase, vacuolar ATP-ase, or acid or alkaline phosphatase. To test the expression strategy, we have made use of pma1-105 (Ser³⁶⁸ → Phe), a vanadate-resistant mutant previously characterized by standard methods (Perlin, D. S., Harris, S. L., Seto-Young, D., and Haber, J. E. (1989) J. Biol. Chem. 264, 21857-21864). In secretory vesicles, as expected, the plasmid-borne pma1-105 allele gives rise to a mutant enzyme with a reduced rate of ATP hydrolysis and a 100-fold increase in K_i for vanadate. Proton pumping is similarly resistant to vanadate. Thus, the vesicles appear well suited for the production and characterization of mutant forms of the PMA1 [H⁺]ATPase. They should also aid the study of other yeast membrane proteins that are essential for growth as well as heterologous proteins whose appearance in the plasma membrane may be toxic to the cell.