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Selective packaging of cargo molecules into endoplasmic reticulum-derived COPII vesicles
1Department of Molecular and Cell Biology, and Howard Hughes Medical Institute, University of California, Berkeley 94720-3202, USA.
Summary
Coat protein II (COPII) vesicles mediate protein transport from the ER to the Golgi. This study reveals distinct requirements for packaging soluble versus membrane cargo into COPII vesicles, suggesting diverse capture mechanisms.
Area of Science:
- Cell Biology
- Molecular Biology
- Protein Transport
Background:
- Coated vesicles, specifically coat protein II (COPII) vesicles, are essential for anterograde protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus.
- COPII vesicle formation in vitro requires Sar1p, Sec23p/Sec24p, and Sec13p/Sec31p protein fractions.
- Understanding the mechanisms of cargo selection and packaging into COPII vesicles is crucial for elucidating cellular protein trafficking pathways.
Purpose of the Study:
- To investigate the molecular requirements for the packaging of different types of cargo, including v-SNAREs, putative adapters (Emp24p), and soluble secretory proteins, into ER-derived COPII vesicles.
- To determine if distinct signals or adapters are involved in the capture of membrane-associated versus soluble cargo.
- To analyze the role of specific COPII components, such as Sar1p and Sec12p, in cargo packaging.
Main Methods:
- In vitro vesicle budding assay using ER-membrane fractions and purified COPII proteins.
- Analysis of protein packaging into COPII vesicles by titrating individual COPII components (Sar1p, Sec12p).
- Utilizing mutant strains (SEC16) to assess the impact on cargo packaging.
Main Results:
- The cytoplasmic domain of the v-SNARE Sec22p is necessary for its packaging into COPII vesicles.
- Packaging of membrane-associated v-SNAREs and Emp24p requires higher concentrations of Sar1p compared to the soluble protein gp alphaF.
- Overproduction of Sec12p facilitates gp alphaF packaging but not v-SNAREs/Emp24p packaging without exogenous Sar1p.
- Mutations in SEC16 gene disproportionately affect v-SNARE and Emp24p packaging compared to gp alphaF packaging.
Conclusions:
- Distinct mechanisms and potentially diverse signals or adapters are involved in the capture of soluble and membrane cargo into COPII transport vesicles.
- Cargo capture into COPII vesicles is not a uniform process and depends on the nature of the cargo molecule.
- These findings highlight the complexity of the COPII machinery in cargo selection and sorting during ER-to-Golgi transport.