A tomato alternative oxidase protein with altered regulatory properties

Abstract

We have investigated the expression and regulatory properties of the two alternative oxidase (Aox) proteins that are expressed in tomato ( Lycopersicon esculentum L. Mill cv. Sweetie) after storage of green fruit at 4 °C. Four Aox genes were identified in the tomato genome, of which two ( LeAox1a and LeAox1b) were demonstrated to be expressed in cold-treated fruit. The activity and regulatory properties of LeAox1a and LeAox1b were assayed after expression of each protein in yeast cells ( Saccharomyces cerevisiae), proving that each is an active Aox protein. The LeAox1b protein was shown to have altered regulatory properties due to the substitution of a Ser for the highly conserved Cys I residue. LeAox1b could not form inactive disulfide-linked dimers and was activated by succinate instead of pyruvate. This is the first example of a dicot species expressing a natural Cys I/Ser isoform. The implications of the existence and expression of such Aox isoforms is discussed in the light of the hypothesised role for Aox in plant metabolism.