There are a few features that distinguish eukaryotic cells from their less evolved contemporaries, the most notable being the presence of intracellular membrane-bound organelles, such as the endoplasmic reticulum (ER) and Golgi apparatus. Various biomolecules are shuttled between these intracellular compartments by vesicles roughly 100 nanometers in diameter. The vesicles are part of the canonical intracellular trafficking pathways that are defined by the coat proteins that constitute their outer coat.
For example, vesicles that follow the Golgi to ER trafficking route are covered in the coat protein complex I (COPI), ER to Golgi traffic is made up of COPII protein coat, and Golgi to plasma membrane traffic contains clathrin-coated vesicles. Now, researchers from the European Molecular Biology Laboratory (EMBL) have assembled detailed images of the intricate protein-coat that surrounds the Golgi to ER vesicles.
“Until now we could see different elements of the vesicle coat, but not get a complete and detailed picture of the coat assembled onto the vesicle membrane,” explained senior author John Briggs, Ph.D., group leader and senior scientist at EMBL. “This is an important step forward for our understanding of intracellular transportation.”
The findings from this study were published recently in Science through an article entitled “A structure of the COPI coat and the role of coat proteins in membrane vesicle assembly.”
The investigators employed a technique called cryo-electron tomography where samples can be frozen at extremely low temperatures in order to avoid fixing or staining artifacts that can often occur from standard electron microscopy sample preparation. The scientists took images from hundreds of vesicles and combined them to create a 3D image of the COPI coat—allowing them to produce some of the most detailed images to date of a completely assembled vesicle coat.
Once the EMBL team began to analyze the assembled 3D images they found some surprising results. It had been thought that the COPI coat was similar to other vesicles, in that the proteins were assembled in different layers around the vesicle membrane. However, the researchers observed that the proteins in the COPI coat were all interwoven together in one large layer that curved to fit the vesicle membrane.
Specifically, the scientists noted that the COPI coat was made up of repeating units of building blocks they called triads, which contained all of the critical functional elements arranged in a unique 3D structure.
“Our images showed us how the proteins that make up the coat are arranged and it was surprising to discover how different COPI is from, for example, clathrin or COPII coated vesicles,” stated Svetlana Dodonova, predoctoral fellow in Dr. Briggs laboratory and lead author on the current study. “Our next step will be to try to find out how this coat forms and binds to the vesicle membrane and how it arranges itself into such complex shapes.”