Incorrect targeting of plant vacuolar lectins in yeast.

Two soluble vacuolar lectins, seed lectin and DB58, from the legume, Dolichos biflorus, were expressed in Saccharomyces cerevisiae using both low and high copy number plasmids under the control of the GAL1 promoter. When expressed at low levels, these lectins were each secreted by 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 associated with the yeast vacuoles. The differential COOH-terminal processing of these lectins, that results in the production of heteroligomers in plants, did not occur in yeast. Site-directed mutagenesis was employed to produce a construct encoding the shorter subunit of the seed lectin. Expression of this truncated subunit in yeast produced the same results as found with the larger subunit, thus indicating that this modification does not provide the vacuolar targeting signal. The inability of these two vacuolar proteins from different plant tissues to be transported to the yeast vacuole suggests that plants and yeast utilize different signals for targeting soluble vacuolar proteins.