Structure of the complete, membrane-assembled COPII coat reveals a complex interaction network

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The COPII (coat complex II) machinery is essential for vesicular transport from the endoplasmic reticulum (ER) to the Golgi apparatus.
• It comprises five proteins forming a two-layered coat, with the inner layer regulating cargo recruitment and coat assembly, and the outer layer forming cages for membrane curvature.
• Current models emphasize the outer coat's role in membrane curvature generation.

Purpose of the Study:

• To visualize the complete, membrane-assembled COPII coat using advanced imaging techniques.
• To elucidate the detailed interaction network within and between the inner and outer COPII coat layers.
• To investigate the functional significance of these interactions in membrane remodeling and cargo trafficking.

Main Methods:

• Cryo-electron tomography (cryo-ET) to visualize the assembled COPII coat.
• Subtomogram averaging to determine high-resolution structures.
• Genetic and biochemical approaches, including mutagenesis, to assess protein function.

Main Results:

• The complete, membrane-assembled COPII coat structure was visualized, revealing extensive interactions.
• Mutagenesis studies demonstrated that the inner COPII coat alone possesses membrane remodeling capabilities.
• The outer coat provides organizational input, refining the membrane curvature function initiated by the inner coat.

Conclusions:

• The functional roles of the inner and outer COPII coats in membrane remodeling and curvature are distinct from current paradigms.
• The inner coat plays a primary role in membrane remodeling, with the outer coat providing organizational input.
• These findings offer a new structural framework for understanding COPII coat regulation in protein trafficking and secretion.