The prepropeptide of vacuolar aminopeptidase I is necessary and sufficient to target the fluorescent reporter protein GFP to the vacuole of yeast by the Cvt pathway

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Abstract
We have studied the capacity of the prepro amino extension of vacuolar protease leucine aminopeptidase I (API) to target the fluorescent reporter protein GFP to the vacuole of yeast. The preproGFP chimera constructed by extending the amino end of GFP with the prepro‐part of API is rapidly degraded in both wild‐type WCG cells and WCG 11/21a cells deficient in the proteasome. In contrast, the chimera expressed in WCG‐PP cells deficient in both proteasome activity and vacuolar proteinase A accumulates in the vacuole, where it remains stable. Replacement of Gly by Ile‐7, a substitution that prevents folding of the pre‐part into an amphipathic helix and inhibits the targeting of the API precursor to the vacuole, inhibits the targeting of preproGFP to the vacuole. The separated pre‐ and pro‐parts of the API precursor do not target GFP to the vacuole. Targeting of preproGFP to the vacuole is independent of its levels of expression, as the fluorescent protein localizes to the vacuole in cells expressing the protein under the control of both the GAL 1/10 or the API promoter. The preproGFP expressed under both promoters is recovered as monomers from cytosolic cell extracts. PreproGFP expressed under the API promoter is packed into cytoplasmic bodies that penetrate into the vacuolar lumen to release the protein. Altogether our results show that the prepro‐part of the API precursor is necessary and sufficient to target the green fluorescent reporter protein to the vacuole.