High-curvature domains of the ER are important for the organization of ER exit sites in Saccharomyces cerevisiae

Integrated self-organization of transitional ER and early Golgi compartments

COPII coated vesicles bud from an ER domain termed the transitional ER (tER), but the mechanism that clusters COPII vesicles at tER sites is unknown. tER sites are closely associated with early Golgi or pre-Golgi structures, suggesting that the clustering of nascent COPII vesicles could be achieved by tethering to adjacent membranes. This model challenges the prevailing view that COPII vesicles are clustered by a scaffolding protein at the ER surface. Although Sec16 was proposed to serve as such a scaffolding protein, recent data suggest that rather than organizing COPII into higher-order structures, Sec16 acts at the level of individual COPII vesicles to regulate COPII turnover. A plausible synthesis is that tER sites are created by tethering to Golgi membranes and are regulated by Sec16. Meanwhile, the COPII vesicles that bud from tER sites are thought to nucleate new Golgi cisternae. Thus, an integrated self-organization process may generate tER-Golgi units.

Rab GAP cascade regulates dynamics of Ypt6 in the Golgi traffic

The Golgi apparatus functions as the central station of membrane traffic in cells, where newly synthesized proteins moving along the secretory pathway merge with proteins recycled from subsequent membrane organelles such as endosomes. A series of Rab GTPases act consecutively and in concert with the maturation of cis- to-trans cisternae of the Golgi apparatus. Rab GTPases control various steps in intracellular membrane traffic by recruiting downstream effector proteins. Here, we report the dynamics of Ypt6, a yeast member of the Rab GTPase family, which mediates the fusion of vesicles from endosomes at the Golgi apparatus. Ypt6 resides temporarily at the Golgi and dissociates into the cytosol upon arrival of Ypt32, another Rab GTPase functioning in the late Golgi. We found that Gyp6, a putative GTPase-activating protein (GAP) for Ypt6, specifically interacts with Ypt32, most likely as an effector. Disruption of GYP6 or introduction of a Rab-GAP activity-deficient mutation in GYP6 resulted in continual residence of Ypt6 at the Golgi. We propose that Ypt32 acts to terminate endosome-to-Golgi traffic through a Rab-GAP cascade as it does for cis-to-trans intra-Golgi traffic. Simultaneous disruption of GAP for early-acting Rab proteins in the Golgi showed appreciable defects in post-Golgi trafficking, but did not significantly affect cell growth.

MAIGO5 functions in protein export from Golgi-associated endoplasmic reticulum exit sites in Arabidopsis

Plant cells face unique challenges to efficiently export cargo from the endoplasmic reticulum (ER) to mobile Golgi stacks. Coat protein complex II (COPII) components, which include two heterodimers of Secretory23/24 (Sec23/24) and Sec13/31, facilitate selective cargo export from the ER; however, little is known about the mechanisms that regulate their recruitment to the ER membrane, especially in plants. Here, we report a protein transport mutant of Arabidopsis thaliana, named maigo5 (mag5), which abnormally accumulates precursor forms of storage proteins in seeds. mag5-1 has a deletion in the putative ortholog of the Saccharomyces cerevisiae and Homo sapiens Sec16, which encodes a critical component of ER exit sites (ERESs). mag mutants developed abnormal structures (MAG bodies) within the ER and exhibited compromised ER export. A functional MAG5/SEC16A-green fluorescent protein fusion localized at Golgi-associated cup-shaped ERESs and cycled on and off these sites at a slower rate than the COPII coat. MAG5/SEC16A interacted with SEC13 and SEC31; however, in the absence of MAG5/SEC16A, recruitment of the COPII coat to ERESs was accelerated. Our results identify a key component of ER export in plants by demonstrating that MAG5/SEC16A is required for protein export at ERESs that are associated with mobile Golgi stacks, where it regulates COPII coat turnover.

ER exit sites are physical and functional core autophagosome biogenesis components

ERES function is required for assembly of the autophagy machinery immediately downstream of the Atg1 kinase complex and is associated with formation of autophagosomes at every stage of the process. ERES are core components of the autophagy machinery for the biogenesis of autophagosomes.

Autophagy is a central homeostasis and stress response pathway conserved in all eukaryotes. One hallmark of autophagy is the de novo formation of autophagosomes. These double-membrane vesicular structures form around and deliver cargo for degradation by the vacuole/lysosome. Where and how autophagosomes form are outstanding questions. Here we show, using proteomic, cytological, and functional analyses, that autophagosomes are spatially, physically, and functionally linked to endoplasmic reticulum exit sites (ERES), which are specialized regions of the endoplasmic reticulum where COPII transport vesicles are generated. Our data demonstrate that ERES are core autophagosomal biogenesis components whose function is required for the hierarchical assembly of the autophagy machinery immediately downstream of the Atg1 kinase complex at phagophore assembly sites.

Secretory protein biogenesis and traffic in the early secretory pathway

The secretory pathway is responsible for the synthesis, folding, and delivery of a diverse array of cellular proteins. Secretory protein synthesis begins in the endoplasmic reticulum (ER), which is charged with the tasks of correctly integrating nascent proteins and ensuring correct post-translational modification and folding. Once ready for forward traffic, proteins are captured into ER-derived transport vesicles that form through the action of the COPII coat. COPII-coated vesicles are delivered to the early Golgi via distinct tethering and fusion machineries. Escaped ER residents and other cycling transport machinery components are returned to the ER via COPI-coated vesicles, which undergo similar tethering and fusion reactions. Ultimately, organelle structure, function, and cell homeostasis are maintained by modulating protein and lipid flux through the early secretory pathway. In the last decade, structural and mechanistic studies have added greatly to the strong foundation of yeast genetics on which this field was built. Here we discuss the key players that mediate secretory protein biogenesis and trafficking, highlighting recent advances that have deepened our understanding of the complexity of this conserved and essential process.

A model for the self-organization of vesicular flux and protein distributions in the Golgi apparatus

The generation of two non-identical membrane compartments via exchange of vesicles is considered to require two types of vesicles specified by distinct cytosolic coats that selectively recruit cargo, and two membrane-bound SNARE pairs that specify fusion and differ in their affinities for each type of vesicles. The mammalian Golgi complex is composed of 6-8 non-identical cisternae that undergo gradual maturation and replacement yet features only two SNARE pairs. We present a model that explains how distinct composition of Golgi cisternae can be generated with two and even a single SNARE pair and one vesicle coat. A decay of active SNARE concentration in aging cisternae provides the seed for a cis[Formula: see text]trans SNARE gradient that generates the predominantly retrograde vesicle flux which further enhances the gradient. This flux in turn yields the observed inhomogeneous steady-state distribution of Golgi enzymes, which compete with each other and with the SNAREs for incorporation into transport vesicles. We show analytically that the steady state SNARE concentration decays exponentially with the cisterna number. Numerical solutions of rate equations reproduce the experimentally observed SNARE gradients, overlapping enzyme peaks in cis, medial and trans and the reported change in vesicle nature across the Golgi: Vesicles originating from younger cisternae mostly contain Golgi enzymes and SNAREs enriched in these cisternae and extensively recycle through the Endoplasmic Reticulum (ER), while the other subpopulation of vesicles contains Golgi proteins prevalent in older cisternae and hardly reaches the ER.

ER structure and function

The ER forms a contiguous structure of interconnected sheets and tubules that spreads from the nuclear envelope to the cell cortex. Through its attachment to the cytoskeleton, the ER undergoes dynamic rearrangements, such as tubule extension and movement. ER shaping proteins (reticulons and DP1/Yop1p) play key roles in generating and maintaining the unique reticular morphology of the ER. Atlastin and its yeast homologue, Sey1p, mediate homotypic ER membrane fusion, which leads to the formation of new three-way junctions within the polygonal network. At these junctions, the Lunapark protein, Lnp1p, works in conjunction with the reticulons, DP1/Yop1p, and in antagonism to atlastin/Sey1p to maintain the network in a dynamic equilibrium. Defects in ER morphology have been linked to certain neurological disorders.

Super-resolution confocal live imaging microscopy (SCLIM) - Cutting-edge technology in cell biology

Super-resolution confocal live imaging microscopy (SCLIM) we developed provides amazingly high-speed live cell imaging at high space resolution. With this technology we are now able to observe details of membrane traffic events, including behaviors of small vesicles, cisternal maturation of the Golgi apparatus, and membrane segregation within a compartment.

cis-Golgi proteins accumulate near the ER exit sites and act as the scaffold for Golgi regeneration after brefeldin A treatment in tobacco BY-2 cells

The Golgi apparatus forms stacks of cisternae in many eukaryotic cells. However, little is known about how such a stacked structure is formed and maintained. To address this question, plant cells provide a system suitable for live-imaging approaches because individual Golgi stacks are well separated in the cytoplasm. We established tobacco BY-2 cell lines expressing multiple Golgi markers tagged by different fluorescent proteins and observed their responses to brefeldin A (BFA) treatment and BFA removal. BFA treatment disrupted cis, medial, and trans cisternae but caused distinct relocalization patterns depending on the proteins examined. Medial- and trans-Golgi proteins, as well as one cis-Golgi protein, were absorbed into the endoplasmic reticulum (ER), but two other cis-Golgi proteins formed small punctate structures. After BFA removal, these puncta coalesced first, and then the Golgi stacks regenerated from them in the cis-to-trans order. We suggest that these structures have a property similar to the ER-Golgi intermediate compartment and function as the scaffold of Golgi regeneration.

Involvement of the penta-EF-hand protein Pef1p in the Ca2+-dependent regulation of COPII subunit assembly in Saccharomyces cerevisiae

Although it is well established that the coat protein complex II (COPII) mediates the transport of proteins and lipids from the endoplasmic reticulum (ER) to the Golgi apparatus, the regulation of the vesicular transport event and the mechanisms that act to counterbalance the vesicle flow between the ER and Golgi are poorly understood. In this study, we present data indicating that the penta-EF-hand Ca²⁺-binding protein Pef1p directly interacts with the COPII coat subunit Sec31p and regulates COPII assembly in Saccharomyces cerevisiae. ALG-2, a mammalian homolog of Pef1p, has been shown to interact with Sec31A in a Ca²⁺-dependent manner and to have a role in stabilizing the association of the Sec13/31 complex with the membrane. However, Pef1p displayed reversed Ca²⁺ dependence for Sec13/31p association; only the Ca²⁺-free form of Pef1p bound to the Sec13/31p complex. In addition, the influence on COPII coat assembly also appeared to be reversed; Pef1p binding acted as a kinetic inhibitor to delay Sec13/31p recruitment. Our results provide further evidence for a linkage between Ca²⁺-dependent signaling and ER-to-Golgi trafficking, but its mechanism of action in yeast seems to be different from the mechanism reported for its mammalian homolog ALG-2.

Insights into structural and regulatory roles of Sec16 in COPII vesicle formation at ER exit sites

Sec16 is an essential factor in “ER exit site” formation, as well as in COPII-mediated traffic in vivo. The results indicate that purified Sec16 alone can self-assemble into homo-oligomeric complexes on a planar lipid membrane and plays an important role in regulating Sar1 GTPase activity at the late steps of COPII vesicle formation.

COPII-coated buds are formed at endoplasmic reticulum exit sites (ERES) to mediate ER-to-Golgi transport. Sec16 is an essential factor in ERES formation, as well as in COPII-mediated traffic in vivo. Sec16 interacts with multiple COPII proteins, although the functional significance of these interactions remains unknown. Here we present evidence that full-length Sec16 plays an important role in regulating Sar1 GTPase activity at the late steps of COPII vesicle formation. We show that Sec16 interacts with Sec23 and Sar1 through its C-terminal conserved region and hinders the ability of Sec31 to stimulate Sec23 GAP activity toward Sar1. We also find that purified Sec16 alone can self-assemble into homo-oligomeric complexes on a planar lipid membrane. These features ensure prolonged COPII coat association within a preformed Sec16 cluster, which may lead to the formation of ERES. Our results indicate a mechanistic relationship between COPII coat assembly and ERES formation.