Figure 1.

Membranes and cargo proteins leave the endoplasmic reticulum (ER), leaving behind transport vesicles packed with proteins by attaching themselves to the COPII coat. This binding involves exit signals on membrane proteins being trapped by the COPII coat and binding with it via their cytosolic tails; some membrane proteins trapped by it then act as cargo receptors to help package other proteins into transport vesicles for transportation. Membrane fusion and the formation of a continuous compartment involve interactions among matching v-SNAREs and t-SNAREs on adjacent identical membranes, beginning with the NSF separation of identical pairs on both membranes displaying matching SNAREs. Thereafter, homotypic fusion expands further by joining with similar-typed vesicles that display matching SNAREs. Once transport vesicles have left an ER exit site and shed their outer layers, they begin merging together. This process, known as homotypic fusion, involves membranes from within one compartment joining forces as opposed to heterotypic fusion, where separate compartments combine; it differs from heterotypic fusion, in which different compartments come together through merging. Like heterotypic fusion, homotypic fusion also requires two corresponding SNAREs, but unlike heterotypic fusion, both membranes contribute v-SNAREs and t-SNAREs, thus creating an even greater level of interaction than heterotypic fusion does.