ER to Golgi Transport :: Cell-biology

COPII Coated Vesicles

Regulated assembly of coat protein complexes (COPs) mediates vesicular transport of materials to and from donor and acceptor membranes [1]. Their specific coats have two main functions; structuring the curved membrane to form a budding vesicle, and as a mode of selecting the vesicle's cargo. They're made of two distinct protein layers. An outer scaffold layer provides a framework for the coat, and an inner layer of adaptors bind the cargo [2]. Coat proteins create shells around forming vesicles via use of small GTPases to direct coat assembly at the donor membrane. There are distinct sets of coat proteins and GTPases used depending on the cargo protein's position in the secretory pathway [1]. Various types of cargo must be carried by vesicles to target compartments within the cell, including secretory, lysosomal and membrane proteins and the machinery needed to target and dock the vesicle to the correct acceptor membrane.[2]

COPII coated vesicles carry cargo from the endoplasmic reticulum (ER) to the ER-Golgi intermediate compartment (ERGIC) and the cis Golgi network (CGN), also known as anterograde forward transport. The types of proteins selected by COPII are enzymes of later stages in the biosynthetic pathway, membrane proteins for docking and fusion of the vesicle to target compartments, and membrane proteins able to bind soluble cargo [2]. In eukaryotes, COPII vesicle mediated protein export is usually carried out at the transitional ER (tER) or ER exit sites (ERES) [1].

COPII coat assembly is initiated by a Ras-like GTPase called Sar1. Sec12 is an ER transmembrane guanine exchange factor (GEF), that activates Sar1 on the ER membrane. Activation of Sar1 occurs when Sec12 phosphorylates a guanosine diphosphate (GDP), bound to inactive Sar1, to guanosine triphosphate (GTP). This GDP to GTP transition exposes an N-terminal α-helix of Sar1, which inserts into the ER membrane [3]. Once membrane bound, Sar1 recruits and binds Sec23 as part of a Sec23/Sec24 curved heterodimer, giving the budding vesicle its curved scaffolding. This complex selects its cargo to form a bigger pre-budding complex which recruits a Sec13/Sec31 heterotetramer. Sec13/Sec31 cross-links with the complexes, driving membrane deformation to form COPII vesicles of 60-70nm in diameter1.

Sec24 of the Sec23/Sec24 heterodimer binds cargo, driven by ER export signals within amino acid sequences on cytoplasmic regions of secretory proteins. Sec24 contains 3+ possible independent signal binding sites on its membrane proximal surface, as well as many subtypes of Sec23 able to pair with Sec24 to bind varying export signals to bind many different cargos. Most transmembrane protein export signals interact with the coat subunits themselves, whereas some transmembrane and soluble cargo proteins bind indirectly with the coat via use of cargo adaptors/receptors [1].

The export of soluble luminal cargo requires unique transmembrane receptor proteins to control lumen cargo interactions with COPII subunits of the cytoplasm. Several transmembrane cargo export signals have been identified. These include a di-acidic, cytoplasmically exposed motif (D/E)x(D/E), dihydrophobic motifs with two hydrophobic residues adjacent to one another at the C-terminus, tyrosine-containing motifs at the C-terminus etc [1]. ERGIC-53 is a transmembrane protein with a long N-terminal luminal domain and a 12-residue cytosolic tail ending in a dihydrophobic phenylalanine (FF) motif. It recognises soluble cargo destined for the ERGIC, functioning as a cargo receptor for blood coagulation factors V and VIII3 and Capethsin C and Z glycoproteins. Once cargo is selected and vesicle budded, the COPII coated vesicles start transport to target compartments [1].