Stage-specific assays to study biosynthetic cargo selection and role of SNAREs in export from the endoplasmic reticulum and delivery to the Golgi

TFG facilitates outer coat disassembly on COPII transport carriers to promote tethering and fusion with ER-Golgi intermediate compartments

The conserved coat protein complex II (COPII) mediates the initial steps of secretory protein trafficking by assembling onto subdomains of the endoplasmic reticulum (ER) in two layers to generate cargo-laden transport carriers that ultimately fuse with an adjacent ER-Golgi intermediate compartment (ERGIC). Here, we demonstrate that Trk-fused gene (TFG) binds directly to the inner layer of the COPII coat. Specifically, the TFG C terminus interacts with Sec23 through a shared interface with the outer COPII coat and the cargo receptor Tango1/cTAGE5. Our findings indicate that TFG binding to Sec23 outcompetes these other associations in a concentration-dependent manner and ultimately promotes outer coat dissociation. Additionally, we demonstrate that TFG tethers vesicles harboring the inner COPII coat, which contributes to their clustering between the ER and ERGIC in cells. Together, our studies define a mechanism by which COPII transport carriers are retained locally at the ER/ERGIC interface after outer coat disassembly, which is a prerequisite for fusion with ERGIC membranes.

Intracellular trafficking and secretion of VLDL

Steady increase in the incidence of atherosclerosis is becoming a major concern not only in the United States but also in other countries. One of the major risk factors for the development of atherosclerosis is high concentrations of plasma low-density lipoprotein, which are metabolic products of very low-density lipoprotein (VLDL). VLDLs are synthesized and secreted by the liver. In this review, we discuss various stages through which VLDL particles go from their biogenesis to secretion in the circulatory system. Once VLDLs are synthesized in the lumen of the endoplasmic reticulum, they are transported to the Golgi. The transport of nascent VLDLs from the endoplasmic reticulum to Golgi is a complex multistep process, which is mediated by a specialized transport vesicle, the VLDL transport vesicle. The VLDL transport vesicle delivers VLDLs to the cis-Golgi lumen where nascent VLDLs undergo a number of essential modifications. The mature VLDL particles are then transported to the plasma membrane and secreted in the circulatory system. Understanding of molecular mechanisms and identification of factors regulating the complex intracellular VLDL trafficking will provide insight into the pathophysiology of various metabolic disorders associated with abnormal VLDL secretion and identify potential new therapeutic targets.

The biogenesis of chylomicrons

The absorption of dietary fat is of increasing concern given the rise of obesity not only in the United States but throughout the developed world. This review explores what happens to dietary fat within the enterocyte. Absorbed fatty acids and monoacylglycerols are required to be bound to intracellular proteins and/or to be rapidly converted to triacylglycerols to prevent cellular membrane disruption. The triacylglycerol produced at the level of the endoplasmic reticulum (ER) is either incorporated into prechylomicrons within the ER lumen or shunted to triacylglycerol storage pools. The prechylomicrons exit the ER in a specialized transport vesicle in the rate-limiting step in the intracellular transit of triacylglycerol across the enterocyte. The prechylomicrons are further processed in the Golgi and are transported to the basolateral membrane via a separate vesicular system for exocytosis into the intestinal lamina propria. Fatty acids and monoacylglycerols entering the enterocyte via the basolateral membrane are also incorporated into triacylglycerol, but the basolaterally entering lipid is much more likely to enter the triacylglycerol storage pool than the lipid entering via the apical membrane.

Purification of active HOPS complex reveals its affinities for phosphoinositides and the SNARE Vam7p

Coupling of Rab GTPase activation and SNARE complex assembly during membrane fusion is poorly understood. The homotypic fusion and vacuole protein sorting (HOPS) complex links these two processes: it is an effector for the vacuolar Rab GTPase Ypt7p and is required for vacuolar SNARE complex assembly. We now report that pure, active HOPS complex binds phosphoinositides and the PX domain of the vacuolar SNARE protein Vam7p. These binding interactions support HOPS complex association with the vacuole and explain its enrichment at the same microdomains on docked vacuoles as phosphoinositides, Ypt7p, Vam7p, and the other SNARE proteins. Concentration of the HOPS complex at these microdomains may be a key factor for coupling Rab GTPase activation to SNARE complex assembly.