COPII-cargo interactions direct protein sorting into ER-derived transport vesicles

Area of Science:

• Cell Biology
• Molecular Biology
• Protein Trafficking

Background:

• Coat protein complex II (COPII) mediates transport from the endoplasmic reticulum (ER) to the Golgi.
• Understanding COPII's role in cargo selection is crucial for cellular protein trafficking.

Purpose of the Study:

• To investigate the function of COPII coat proteins in cargo selection and recruitment.
• To identify the molecular components involved in cargo packaging into COPII vesicles.

Main Methods:

• Isolation of integral membrane and soluble cargo proteins in complexes with COPII components (Sar1, Sec23/24) and GTP analogs.
• Analysis of protein inclusion in COPII complexes, including the role of specific proteins like Shr3.
• Isolation of COPII-cargo and adaptor-cargo complexes from COPII vesicles.

Main Results:

• Sar1 and Sec23/24, with GTP or GMP-PNP, formed complexes with integral and soluble cargo proteins.
• Vesicle fusion proteins (vSNAREs) and Emp24 were found in COPII complexes.
• Shr3 was essential for the inclusion of amino-acid permease cargo.
• Resident ER proteins (Sec61, BiP, Shr3) were excluded, indicating specific cargo binding.
• COPII-cargo and adaptor-cargo complexes were isolated from vesicles.

Conclusions:

• COPII components, specifically Sar1-GTP and Sec23/24, recognize cargo packaging signals and soluble cargo adaptors.
• The COPII machinery ensures selective transport of cargo from the ER by binding specifically to destined molecules.
• Shr3 acts as a specific factor for the ER export of certain permeases.