COPII

Abstract

Vesicular traffic provides a dynamic and elaborate communication network between the subcellular compartments that define the structure and identity of membrane-bound organelles (Bonifacino and Glick 2004). The molecular and structural mechanisms that direct lipid and protein cargo flow between discontinuous subcellular organelles involve specialized multi-protein machineries that are defined by the molecular and structural properties of cytosolic coat protein complexes (0Bonifacino and Lippincott-Schwartz 2003). The Golgi apparatus is certainly involved in this flow. The endoplasmicreticulum (ER) is responsible for the synthesis of the proteins of most of the cellular organelles. Newly synthesized secretory proteins are translated at the rough ER and translocated into the ER lumen or ER membrane through the translocation channel, where they undergo folding, assembly and post-translational modifications with the aid of a variety of ER chaperones. Correctly folded and assembled secretory proteins are then segregated from ER resident proteins and transported to the Golgi apparatus for further processing and secretion. The ER-to-Golgi transport step is thought to occur via membrane-bound vesicles or carrier intermediates, which are formed by the assembly of the coat protein complex II (COPII) on the ER membranes. COPII is the name given to a cytosolic protein complex required for direct capture of cargo molecules and for the physical deformation of the ER membrane that drives the formation of the so-called COPII vesicles or carrier intermediates in anterograde transport from the ER to the Golgi. Protein export by COPII vesicle from the ER is the default ER-to-Golgi route that has been proposed in yeast and mammals. Cargo proteins that are destined for delivery to the Golgi apparatus need not only refer to newly synthesized biosynthetic cargo molecules, but also a variety of other machinery proteins that constantly cycle between the ER and the Golgi are included.