Secretory cargo regulates the turnover of COPII subunits at single ER exit sites

Sec16 defines endoplasmic reticulum exit sites and is required for secretory cargo export in mammalian cells

The selective export of proteins and lipids from the endoplasmic reticulum (ER) is mediated by the coat protein complex II (COPII) that assembles onto the ER membrane. In higher eukaryotes, COPII proteins assemble at discrete sites on the membrane known as ER exit sites (ERES). Here, we identify Sec16 as the protein that defines ERES in mammalian cells. Sec16 localizes to ERES independent of Sec23/24 and Sec13/31. Overexpression, and to a lesser extent, small interfering RNA depletion of Sec16, both inhibit ER-to-Golgi transport suggesting that Sec16 is required in stoichiometric amounts. Sar1 activity is required to maintain the localization of Sec16 at discrete locations on the ER membrane, probably through preventing its dissociation. Our data suggest that Sar1-GTP-dependent assembly of Sec16 on the ER membrane forms an organized scaffold defining an ERES.

The Ca2+-binding protein ALG-2 is recruited to endoplasmic reticulum exit sites by Sec31A and stabilizes the localization of Sec31A

The formation of transport vesicles that bud from endoplasmic reticulum (ER) exit sites is dependent on the COPII coat made up of three components: the small GTPase Sar1, the Sec23/24 complex, and the Sec13/31 complex. Here, we provide evidence that apoptosis-linked gene 2 (ALG-2), a Ca(2+)-binding protein of unknown function, regulates the COPII function at ER exit sites in mammalian cells. ALG-2 bound to the Pro-rich region of Sec31A, a ubiquitously expressed mammalian orthologue of yeast Sec31, in a Ca(2+)-dependent manner and colocalized with Sec31A at ER exit sites. A Ca(2+) binding-deficient ALG-2 mutant, which did not bind Sec31A, lost the ability to localize to ER exit sites. Overexpression of the Pro-rich region of Sec31A or RNA interference-mediated Sec31A depletion also abolished the ALG-2 localization at these sites. In contrast, depletion of ALG-2 substantially reduced the level of Sec31A associated with the membrane at ER exit sites. Finally, treatment with a cell-permeable Ca(2+) chelator caused the mislocalization of ALG-2, which was accompanied by a reduced level of Sec31A at ER exit sites. We conclude that ALG-2 is recruited to ER exit sites via Ca(2+)-dependent interaction with Sec31A and in turn stabilizes the localization of Sec31A at these sites.

COPII-Golgi protein interactions regulate COPII coat assembly and Golgi size

Under experimental conditions, the Golgi apparatus can undergo de novo biogenesis from the endoplasmic reticulum (ER), involving a rapid phase of growth followed by a return to steady state, but the mechanisms that control growth are unknown. Quantification of coat protein complex (COP) II assembly revealed a dramatic up-regulation at exit sites driven by increased levels of Golgi proteins in the ER. Analysis in a permeabilized cell assay indicated that up-regulation of COPII assembly occurred in the absence GTP hydrolysis and any cytosolic factors other than the COPII prebudding complex Sar1p–Sec23p–Sec24p. Remarkably, acting via a direct interaction with Sar1p, increased expression of the Golgi enzyme N-acetylgalactosaminyl transferase-2 induced increased COPII assembly on the ER and an overall increase in the size of the Golgi apparatus. These results suggest that direct interactions between Golgi proteins exiting the ER and COPII components regulate ER exit, providing a variable exit rate mechanism that ensures homeostasis of the Golgi apparatus.

Membrane deformation by protein coats

Protein coats deform lipid membranes into spherical buds, which undergo fission at the neck to become vesicles. To induce membrane curvature, protein coats use basic tools including amphipathic helices and concave protein surfaces, and take advantage of the bulk properties of cellular membranes, such as loose lipid packing in the endoplasmic reticulum and cis-Golgi and the abundance of anionic lipids in the cytosolic leaflet of the plasma membrane. Protein scaffolds, sensors of membrane curvature and finely tuned reactions such as GTP hydrolysis permit the spatial and temporal organization of these tools, making protein coats self-organized molecular machines. Because biological membranes generally adhere to a cytoskeleton, the functioning of protein coats is coupled to other large remodeling events at the membrane interface.

Seeking a way out: export of proteins from the plant endoplasmic reticulum

The functionality of the secretory pathway relies on the efficient transfer of cargo molecules from their site of synthesis in the endoplasmic reticulum (ER) to successive compartments within the pathway. Although transport mechanisms of secretory proteins have been studied in detail in various non-plant systems, it is only recently that our knowledge of secretory routes in plants has expanded dramatically. This review focuses on exciting new findings concerning the exit mechanisms of cargo proteins from the plant ER and the role of ER export sites in this process.

Sterols regulate ER-export dynamics of secretory cargo protein ts-O45-G

Alterations in endoplasmic reticulum (ER) cholesterol are fundamental for a variety of cellular processes such as the regulation of lipid homeostasis or efficient protein degradation. We show that reduced levels of cellular sterols cause a delayed ER-to-Golgi transport of the secretory cargo membrane protein ts-O45-G and a relocation to the ER of an endogenous protein cycling between the ER and the Golgi complex. Transport inhibition is characterized by a delay in the accumulation of ts-O45-G in ER-exit sites (ERES) and correlates with a reduced mobility of ts-O45-G within ER membranes. A simple mathematical model describing the kinetics of ER-exit predicts that reduced cargo loading to ERES and not the reduced mobility of ts-O45-G accounts for the delayed ER-exit and arrival at the Golgi. Consistent with this, membrane turnover of the COPII component Sec23p is delayed in sterol-depleted cells. Altogether, our results demonstrate the importance of sterol levels in COPII mediated ER-export.

Arf GAPs and membrane traffic

The selective transfer of material between membrane-delimited organelles is mediated by protein-coated vesicles. In many instances, formation of membrane trafficking intermediates is regulated by the GTP-binding protein Arf. Binding and hydrolysis of GTP by Arf was originally linked to the assembly and disassembly of vesicle coats. Arf GTPase-activating proteins (GAPs), a family of proteins that induce hydrolysis of GTP bound to Arf, were therefore proposed to regulate the disassembly and dissociation of vesicle coats. Following the molecular identification of Arf GAPs, the roles for GAPs and GTP hydrolysis have been directly examined. GAPs have been found to bind cargo and known coat proteins as well as directly contribute to vesicle formation, which is consistent with the idea that GAPs function as subunits of coat proteins rather than simply Arf inactivators. In addition, GTP hydrolysis induced by GAPs occurs largely before vesicle formation and is required for sorting. These results are the primary basis for modifications to the classical model for the function of Arf in transport vesicle formation, including a recent proposal that Arf has a proofreading, rather than a structural, role.

Toward the Systems Biology of Vesicle Transport

Systems biology aims to study complex biological processes, such as intracellular traffic, as a whole. Systematic genome-wide assays have the potential to identify the transport machinery, delineate pathways and uncover the molecular components of physiological processes that influence trafficking. A goal of this approach is to create predictive models of intracellular trafficking pathways that reflect these relationships. In this review, we highlight current genome-wide technologies of particular relevance to vesicle transport and describe recent applications of these technologies in the framework of systems biology. Systems approaches hold great promise for placing trafficking pathways in their cellular contexts.