Transport of aminopeptidase I (API) to the vacuole appears to be insensitive to blockage of the secretory pathway. Here we show that the N‐terminal extension of the 61 kDa precursor of API (pAPI) is proteolytically processed in two sequential steps. The first step involves proteinase A (PrA) and produces a 55 kDa unstable intermediate (iAPI). The second step involves proteinase B (PrB) and converts iAPI into the 50 kDa stable, mature enzyme (mAPI). Reversion of the cup1 growth phenotype by a pAPI‐CUP1 chimera indicates that pAPI is transported to the vacuole by a post‐translational mechanism. Deletion of the first 16 amino acids results in accumulation of the truncated protein in the cytosol, indicating that pAPI is actively transported to the vacuole. The chimera pAPI‐myc, constructed by fusing a myc tag to the C‐terminus of pAPI, was exploited to dissect the mechanism of pAPI transport. Cell fractionation studies show the presence of iAPI‐myc and mAPI in a fraction of vacuoles purified by density centrifugation. This and the sequential conversion of pAPI‐myc into iAPI‐myc and mAPI lacking the myc tag is consistent with insertion of pAPI into the vacuolar membrane through its N‐terminal extension. The specific mechanism of API sorting demonstrates a new pathway of protein transport in vacuolar biogenesis.

Uteroglobin, a progesterone-binding secretory protein, was shown to bind retinoic acid and retinol in a non-saturable manner, at least up to concentrations of retinoids of 20 PM. Binding is increased about IO-fold by previous reduction of uteroglobin with 10 mM dithothreitol and it is not affected by previous saturation of the progesterone binding site, suggesting different binding sites for the steroid and the retinoids. The results are discussed in relation to a possible physiological role for this protein.

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