Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 1997 Jul 14;138(1):37-44.
doi: 10.1083/jcb.138.1.37.

Aminopeptidase I is targeted to the vacuole by a nonclassical vesicular mechanism

S V Scott  1 , M Baba, Y Ohsumi, D J Klionsky
Affiliations

Aminopeptidase I is targeted to the vacuole by a nonclassical vesicular mechanism

S V Scott et al. J Cell Biol. .

Abstract

The yeast vacuolar protein aminopeptidase I (API) is synthesized as a cytosolic precursor that is transported to the vacuole by a nonclassical targeting mechanism. Recent genetic studies indicate that the biosynthetic pathway that transports API uses many of the same molecular components as the degradative autophagy pathway. This overlap coupled with both in vitro and in vivo analysis of API import suggested that, like autophagy, API transport is vesicular. Subcellular fractionation experiments demonstrate that API precursor (prAPI) initially enters a nonvacuolar cytosolic compartment. In addition, subvacuolar vesicles containing prAPI were purified from a mutant strain defective in breakdown of autophagosomes, further indicating that prAPI enters the vacuole inside a vesicle. The purified subvacuolar vesicles do not appear to contain vacuolar marker proteins. Immunogold EM confirms that prAPI is localized in cytosolic and in subvacuolar vesicles in a mutant strain defective in autophagic body degradation. These data suggest that cytosolic vesicles containing prAPI fuse with the vacuole to release a membrane-bounded intermediate compartment that is subsequently broken down, allowing API maturation.

PubMed Disclaimer

Figures

Figure 2
Figure 2
Precursor API is trapped in a prevacuolar compartment in vps18 cells. (A) Yeast strain JSR18Δ1 with plasmid pJSR9 (vps18 ts) was grown at the permissive temperature of 26°C. Before labeling, the cells were incubated at either 26° or 38°C for 10 min. Pulse labeling was for 5 min, followed by the indicated chase times. (B) vps18 spheroplasts were shifted to 38°C for 5 min, pulse labeled for 5 min, and then chased for either 0 or 60 min. At each time point the spheroplasts were collected by centrifugation and resuspended in PS0 buffer. The Total (T) fraction either received no treatment, was treated with 50 μg/ml proteinase K, or received proteinase K in the presence of 0.2% Triton X-100. The supernatant (S0) and pellet (P0) fractions were collected after centrifugation at 10,000 g for 5 min. (C) vps18 spheroplasts were pulse labeled for 10 min, chased for 10 min at 26°C, and then shifted to 38°C and chased for an additional 60 min. These cells were then lysed in PS200 buffer, and the S200 and P200 fractions were collected by centrifugation at 5,000 g for 5 min. The P200 fraction was resuspended in 10% Ficoll solution made in PS200, and vacuoles were isolated by flotation through 4% Ficoll. V, vacuole fraction; I, 4/10% Ficoll interface fraction; P, gradient pellet. Proteins of interest were recovered by immunoprecipitation, followed by SDS-PAGE, and detected by a Molecular Dynamics STORM phosphorimager. The positions of precursor and mature API and CPY are indicated.
Figure 3
Figure 3
A mutant in the API propeptide causes accumulation of prAPI on a nonvacuolar compartment. (A) Spheroplasts from an ape1Δ strain containing a plasmid bearing P22L API were subjected to differential lysis in PS200 buffer. The Total (T) fraction (after differential lysis) either received no treatment, was treated with 50 μg/ ml proteinase K, or received proteinase K in the presence of 0.2% Triton X-100. (B) The supernatant (S200) and pellet (P200) fractions were collected after centrifugation at 5,000 g for 5 min. The P200 fraction was resuspended in 10% Ficoll, and vacuoles were isolated by flotation through 4% Ficoll. V, vacuole; I, 4/10% Ficoll interface; P, gradient pellet. Proteins were detected by Western blotting. The positions of prAPI, mAPI, and mPrA are indicated. (C) ape1Δ cells containing P22L API on a plasmid were pulse labeled for 5 min, harvested by centrifugation, and subjected to a nonradioactive chase in either nitrogen-containing (SMD) or nitrogen starvation (SD-N) medium. Samples were collected at the times indicated and immunoprecipitated with API antiserum. The resulting SDS-PAGE gels were quantified using a Molecular Dynamics STORM phosphorimager.
Figure 1
Figure 1
API delivery is blocked at low temperatures. SEY6210 cells were pulse labeled for 5 min and chased for the times indicated. Identical experiments were performed at 10°, 14°, and 30°C. API was recovered by immunoprecipitation followed by SDS-PAGE and detected by a STORM phosphorimager (Molecular Dynamics, Sunnyvale, CA). The positions of prAPI and mAPI are indicated.
Figure 4
Figure 4
Precursor API is trapped within a vesicle in cvt17 mutants. (A) Precursor API is in a protease-protected compartment in cvt17 mutants. THY32 (cvt17) spheroplasts were subjected to differential lysis and protease treatment in PS200 buffer as in Fig 3. (B) Precursor API is not free in the vacuolar lumen in cvt17 mutants. Differential fractionation was continued by resuspending the P200 fraction in PS0 buffer and separating the supernatant (S0) and pellet (P0) fractions by centrifugation at 10,000 g. The S0 and P0 fractions were treated with proteinase K in the presence and absence of Triton X-100 as indicated. All fractions were precipitated with 10% TCA and resolved by SDS PAGE; proteins of interest were detected by Western blotting. The positions of prAPI, mAPI, and mPrA are indicated.
Figure 5
Figure 5
Precursor API is contained within subvacuolar vesicles in cvt17 mutants. (A) Isolated vacuoles (V) from cvt17 cells were lysed in PS0 buffer, loaded on top of an Optiprep step gradient, and centrifuged at 170,000 g for 60 min as described in Materials and Methods. Fractions VV, 0/1.06 interface; LD, 1.06 region; SV, 1.06/1.12 interface; HD, 1.12 region. (B) An aliquot of fractions VV and SV was treated with proteinase K either in the presence or absence of Triton X-100 as indicated. Because of the presence of protease inhibitors in the vesicle isolation procedure, an intermediate-sized API degradation product results from proteinase K treatment. The V fraction represents one-tenth of the load of the Optiprep gradient. Proteins were detected by Western blotting. The positions of prAPI and mALP are indicated.
Figure 6
Figure 6
Immuno-EM of cvt17 cells. Yeast strains SEY6210 (parental wild type) and THY32 (cvt17) were grown in YPD and prepared for microscopy as described in Materials and Methods. (A) Wild-type cells probed with antibody against mature API. (B) A cvt17 section showing a cytosolic vesicle containing prAPI probed with antibody against the API proregion. (C) A cvt17 section showing subvacuolar vesicles containing API probed with antibody against mature API. N, nucleus; V, vacuole; (arrows) API containing vesicles. Bars, 0.5 μm.
Figure 7
Figure 7
Characterization of API-containing vesicles. (A) Fractionations were performed as in Fig. 5. The total (T), vacuole (V), vacuolar vesicle (VV), and subvacuolar vesicle (SV) fractions are shown. Proteins were detected by immunoblotting as indicated in the figure. (B) SYPRO orange staining detected by fluorescent scanning mode on a Molecular Dynamics STORM phosphorimager. The relative mobility of the molecular mass standards are indicated on the left. Polypeptides enriched in the subvacuolar vesicles are indicated on the right.
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

See all "Cited by" articles

References

    1. Baba M, Takeshige K, Baba N, Ohsumi Y. Ultrastructural analysis of the autophagic process in yeast: detection of autophagosomes and their characterization. J Cell Biol. 1994;124:903–913. - PMC - PubMed
    1. Baba M, Osumi M, Ohsumi Y. Analysis of the membrane structures involved in autophagy in yeast by freeze-replica method. Cell Struct Funct. 1995;20:465–471. - PubMed
    1. Baker D, Schekman R. Reconstitution of protein transport using broken yeast spheroplasts. Methods Cell Biol. 1989;31:127–141. - PubMed
    1. Barlowe C, Orci L, Yeung T, Hosobuchi M, Hamamoto S, Salama N, Rexach MF, Ravazzola M, Amherdt M, Schekman R. COPII: a membrane coat formed by Sec proteins that drive vesicle budding from the endoplasmic reticulum. Cell. 1994;77:895–907. - PubMed
    1. Conibear E, Stevens TH. Vacuolar biogenesis in yeast: sorting out the sorting proteins. Cell. 1995;83:513–516. - PubMed

Publication types

MeSH terms