Functional role of positively charged amino acid residues in the transmembrane segments of the yeast PMA1 ATPase

Point mutations in the different domains of the Saccharomyces cerevisiae plasma membrane PMA1 ATPase cause redistribution among fractions of inorganic polyphosphates

Both ATP and inorganic polyphosphates (PolyP) appeared to be involved in the yeast energy homeostasis, in which plasma membrane PMA1 H⁺-АТРase plays one of the key roles. During biogenesis and functioning, the enzyme undergoes structural and regulatory phosphorylation. Aim of the work was to elucidate interconnection between functioning of the yeast PMA1 H⁺-АТРase carrying point substitutions that affected the enzyme structure-function relationship and its ability to be phosphorylated and PolyP metabolism. Effect of such replacements of phosphorylable and non-phosphorylable residues in three topologically and functionally different domains of the enzyme - membrane, extracytosolic, and C-terminal - on the metabolism of polyphosphates and distribution between short-, mid-, and long-chained PolyP fractions (PolyP1-PolyP4-5) has been studied. АТРase activity of membrane and most extracytosolic strains was noticeably lower comparing to the wild type. Of these mutants, three substitutions (L801A, E803A, E847A) have not caused significant changes in PolyP content regardless up to twofold drop of the ATPase activity; F796A with four-fold decreased activity has led to noticeable increase of mid-chained PolyP fractions. The most pronounced effect of PolyP redistribution was caused either by removal of potential (S846A, T850A, D851A) or established (S911A) phosphosites in the PMA1 ATPase or by altering type of the established phosphosite (S911D, T912D). Patterns of PolyP fractions for these two groups have significantly differed from each other, occurring in opposite directions for mutants with removed and changed phosphosite. Changing residue of phosphosite without altering its type (T850S) has not led to significant changes in PolyP content.Communicated by Ramaswamy H. Sarma.

Point mutations in the extracytosolic loop between transmembrane segments M5 and M6 of the yeast Pma1 H+-ATPase: alanine-scanning mutagenesis

Membrane-spanning segments M4, M5, M6, and M8 of the H(+)-, Ca(2+)-, and K(+), Na(+)-ATPases, which belong to the P2-type pumps are the core through which cations are transported. M5 and M6 loop is a short extracytoplasmic stretch of the seven amino acid residues (714-DNSLDID) connecting two of these segments, M5 and M6, where residues involved in the formation of the proton-binding site(s) are located. In the present study, we have used alanine-scanning mutagenesis to explore the structural and functional relationships within this loop of the yeast plasma membrane Pma1 H(+)-ATPase. Of the 7 Ala mutants made, substitution for the most conserved residue (Leu-717) has led to a severe misfolding and complete block in biogenesis of the mutant enzyme. The replacement of Asp-714 has also caused misfolding leading to significant decrease in the expression of the mutant and loss of activity. The remaining mutants were expressed in secretory vesicles at 21-119% of the wild-type level and were active enough to be analyzed in detail. One of these mutants (I719A) showed five- to threefold decrease in both expression and ATP hydrolyzing and H(+) pumping activities and also threefold reduction in the coupling ratio between ATP hydrolysis and H(+) transport. Thus, Ala substitutions at three positions of the seven seriously affected biogenesis, folding, stability and/or functioning of the enzyme. Taken together, these results lead to suggestion that M5 and M6 loop play an important role in the protein stability and function and is responsible for proper arrangement of transmembrane segments M5 and M6 and probably other domains of the enzyme. Results for additional conserved substitutions (Asn and Glu) at Asp-714 and Asp-720 confirmed this suggestion.