Transport of a sweet potato storage protein, sporamin, to the vacuole in yeast cells

The propeptide of a precursor to sporamin, a storage protein of sweet potato, is required for targeting of sporamin to the vacuole in transformed tobacco cells (Matsuoka and Nakamura 1991). A fusion gene consisting of an inducible GAL 10 promoter and sporamin cDNA was introduced into Saccharomyces cerevisiae by use either of a multiple-copy plasmid (YEpSAD16) or of a single-copy plasmid (YCpSAD16) to control the level of expression of the precursor. Although we could not detect any sporamin-related polypeptides in cells that harbored YCpSAD16, extracts from cells that harbored YEpSAD16 contained multiple forms of sporaminrelated polypeptides: preprosporamin, prosporamin and several polypeptides that were smaller than prosporamin. However, YCpSAD16 directed the accumulation of prosporamin in pep4 mutant yeast cells that lack vacuolar proteases, andpep4 mutant cells that harbored YEpSAD16 did not contain any sporamin-related polypeptides smaller than prosporamin. The vacuole fractions isolated from the wild-type and pep4 mutant cells contained sporamin-related polypeptides smaller than prosporamin and prosporamin, respectively. These and other results suggest that, at a low level of expression of the precursor, prosporamin is transported to the vacuole and degraded by vacuolar proteases. A mutant precursor to sporamin, in which the propeptide and the N-terminal region of mature sporamin were replaced by an unrelated sequence of four amino acid residues, directed the secretion of sporamin to the culture medium in transformed tobacco cells. However, this mutation did not affect the transport of sporamin to the vacuole in yeast cells and none of the sporamin-related polypeptides were secreted to the extracellular space.