Effects of C-terminal truncations on trafficking of the yeast plasma membrane H+-ATPase

Within the large family of P-type cation-transporting ATPases, members differ in the number of C-terminal transmembrane helices, ranging from two in Cu2+-ATPases to six in H+-, Na+,K+-, Mg2+-, and Ca2+-ATPases. In this study, yeast Pma1 H+-ATPase has served as a model to examine the role of the C-terminal membrane domain in ATPase stability and targeting to the plasma membrane. Successive truncations were constructed from the middle of the major cytoplasmic loop to the middle of the extended cytoplasmic tail, adding back the C-terminal membrane-spanning helices one at a time. When the resulting constructs were expressed transiently in yeast, there was a steady increase in half-life from 70 min in Pma1 delta452 to 348 min in Pma1 delta901, but even the longest construct was considerably less stable than wild-type ATPase (t(1/2) = 11 h). Confocal immunofluorescence microscopy showed that 11 of 12 constructs were arrested in the endoplasmic reticulum and degraded in the proteasome. The only truncated ATPase that escaped the ER, Pma1 delta901, traveled slowly to the plasma membrane, where it hydrolyzed ATP and supported growth. Limited trypsinolysis showed Pma1 delta901 to be misfolded, however, resulting in premature delivery to the vacuole for degradation. As model substrates, this series of truncations affirms the importance of the entire C-terminal domain to yeast H+-ATPase biogenesis and defines a sequence element of 20 amino acids in the carboxyl tail that is critical to ER escape and trafficking to the plasma membrane.