INCORRECT TARGETING OF PLANT VACUOLAR LECTINS IN YEAST

Two soluble vacuolar lectins, seed lectin and DB58, from the legume, D olichos biflorus, were expressed in Saccharomyces cerevisiae using bot h low and high copy number plasmids under the control of the GAL1 prom oter. When expressed at low levels, these lectins were each secreted b y the yeast at all stages of growth. Expression of the lectins at high levels resulted in the retention of most of the lectins in the cell. Cell fractionation studies showed that this retained lectin was not as sociated with the yeast vacuoles. The differential COOH-terminal proce ssing of these lectins, that results in the production of heteroligome rs in plants, did not occur in yeast. Site-directed mutagenesis was em ployed to produce a construct encoding the shorter subunit of the seed lectin. Expression of this truncated subunit in yeast produced the sa me results as found with the larger subunit, thus indicating that this modification does not provide the vacuolar targeting signal. The inab ility of these two vacuolar proteins from different plant tissues to b e transported to the yeast vacuole suggests that plants and yeast util ize different signals for targeting soluble vacuolar proteins.