The Specificity of Vesicle Trafficking: Coat Proteins and SNAREs

Abstract

Proteins that are destined for the secretory system usually begin their journey at the endoplasmic reticulum (ER), where proteins are translocated across the ER membrane into the lumen, before being selectively removed from the ER and packaged as cargo into transport vesicles bound for the stacks of the Golgi complex. Protein cargo then passes through the Golgi to the trans-Golgi network (TGN), where proteins destined for the vacuole (or, in the case of mammalian cells, the lysosome) are sorted away from cargo intended for secretion or for localization at the plasma membrane. This journey, from the ER to either the cell surface or the vacuole, is referred to as the anterograde pathway. Because protein transport through the endomembrane system is not a one-way trip, however, some vesicles also carry proteins in the reverse direction. This retrograde pathway is essential for the recovery of proteins that may have escaped from other endomembrane compartments and also for the recycling of the machinery involved in anterograde transport (see Battey et al., 1999, in this issue). Other endomembrane compartments are found as intermediate locations between the well-known organelles. For example, cargo destined for the vacuole may first pass through a compartment between the TGN and the vacuole called the prevacuolar compartment (PVC). The PVC is sometimes called the “late endosome” (especially in mammalian cells) or just “endosome” (especially in yeast). Throughout this review, we use the term PVC for this compartment, reserving the term endosome for the initial sorting compartment of the endocytic pathway. The first target for vesicles endocytosed from the cell surface is the endosome. Material within the endosome undergoes sorting such that cargo destined to return to the plasma membrane or continue toward the TGN is separated from cargo that will be degraded in the vacuole. The cargo intended for degradation first passes through the PVC, where it mixes with anterograde cargo that is also in transit to the vacuole. In this way, a single compartment can carry simultaneously both anterograde and retrograde cargo, indicating the intricate connections of the secretory system. A diagram of the compartments of the endomembrane system is shown in Figure 1. With traffic moving in both directions between organelles, how a particular cargo is packaged into the correct transport vesicle and how this vesicle is able to differentiate the correct target membrane from among all the others are important questions. Selection and packaging of cargo at the donor organelle depend on various coat proteins that assemble onto the donor membrane surface and mechanically form the transport vesicle. However, it is not the coat proteins that determine the target of a transport vesicle; instead, this is the role of proteins called SNAREs, which separately reside on the vesicle and target membranes. Together, the coat proteins and the SNAREs coordinate the trafficking of the cargo between the various organelles of the endomembrane system. Transport vesicles are formed at donor organelles through the action of several distinct coat proteins. Each type of coat protein is made of specific “coatomers,” and each is somewhat specific for a particular donor organelle, although some coats function at many different donor membranes. For example, a single type of coat, made from the COPII coatomer, is involved in forming vesicles at the ER, whereas a second coat, made from the COPI coatomer, forms vesicles from the Golgi complex (reviewed in Schekman and Orci, 1996). In contrast, many different types of coated vesicles are formed at the TGN, each with a distinct class of cargo destined for the vacuole, the PVC, or the cell surface. Retrograde transport, either from the cell surface, the endosome, or the PVC, also relies on coated vesicles. Coatomer components generally are recruited to the surface of the donor membrane through the action of a small GTPase that, upon binding GTP, associates with membranes and directs the assembly of the coat. Coatomer assembly mechanically drives the formation of a membrane bud, which subsequently pinches off from the membrane to form the transport vesicle. Cargo, which concentrates in the region of the membrane bud in a poorly understood manner, becomes incorporated into the lumen of the transport vesicle. Coated vesicles are not competent for fusion with the target compartments, so the coat must be depolymerized before fusion with the target membrane. This disassembly is believed to be stimulated by hydrolysis of GTP by the membrane-associated GTPase. The disassembly of the coat probably exposes the targeting machinery on the vesicle surface (i.e., the SNAREs) for subsequent delivery of the cargo to the appropriate organelle. The Endomembrane System of Yeast. The endomembrane system consists of seven major compartments: the endoplasmic reticulum (ER), the Golgi complex, the trans-Golgi network (TGN), the vacuole (or l