Identification of a cytoplasm to vacuole targeting determinant in aminopeptidase I.
Aminopeptidase I (API) is a soluble leucine aminopeptidase resident in the yeast vacuole (Frey, J., and K.H. Rohm. 1978. Biochim. Biophys. Acta. 527:31-41). The precursor form of API contains an amino-terminal 45-amino acid propeptide, which is removed by proteinase B (PrB) upon entry into the vacuole. The propeptide of API lacks a consensus signal sequence and it has been demonstrated that vacuolar localization of API is independent of the secretory pathway (Klionsky, D.J., R. Cueva, and D.S. Yaver. 1992. J. Cell Biol. 119:287-299). The predicted secondary structure for the API propeptide is composed of an amphipathic alpha-helix followed by a beta-turn and another alpha-helix, forming a helix-turn-helix structure. With the use of mutational analysis, we determined that the API propeptide is essential for proper transport into the vacuole. Deletion of the entire propeptide from the API molecule resulted in accumulation of a mature-sized protein in the cytosol. A more detailed examination using random mutagenesis and a series of smaller deletions throughout the propeptide revealed that API localization is severely affected by alterations within the predicted first alpha-helix. In vitro studies indicate that mutations in this predicted helix prevent productive binding interactions from taking place. In contrast, vacuolar import is relatively insensitive to alterations in the second predicted helix of the propeptide. Examination of API folding revealed that mutations that affect entry into the vacuole did not affect the structure of API. These data indicate that the API propeptide serves as a vacuolar targeting determinant at a critical step along the cytoplasm to vacuole targeting pathway.
Oda
M N
MN
Section of Microbiology, University of California, Davis 95616, USA.
Scott
S V
SV
Hefner-Gravink
A
A
Caffarelli
A D
AD
Klionsky
D J
DJ
D000595:Amino Acid Sequence; D000626:Aminopeptidases / Q000737:chemistry / Q000378:metabolism*; D001483:Base Sequence; D001692:Biological Transport; D002451:Cell Compartmentation; D003593:Cytoplasm / Q000378:metabolism*; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism*; D019077:Helix-Turn-Helix Motifs; D007425:Intracellular Membranes / Q000378:metabolism; D008969:Molecular Sequence Data; D016296:Mutagenesis; D021382:Protein Sorting Signals / Q000737:chemistry / Q000378:metabolism*; D017433:Protein Structure, Secondary; D012441:Saccharomyces cerevisiae / Q000201:enzymology* / Q000648:ultrastructure; D029701:Saccharomyces cerevisiae Proteins; D014617:Vacuoles / Q000378:metabolism*
D005656:Fungal Proteins; D021382:Protein Sorting Signals; D029701:Saccharomyces cerevisiae Proteins; D000626:Aminopeptidases; C019689:APE1 protein, S cerevisiae