Membrane Transport at an Organelle Interface in the Early Secretory Pathway: Take Your Coat Off and Stay a While: Evolution of the metazoan early secretory pathway

Abstract

Most metazoan organisms have evolved a mildly acidified and calcium diminished sorting hub in the early secretory pathway commonly referred to as the Endoplasmic Reticulum-Golgi intermediate compartment (ERGIC). These membranous vesicular-tubular clusters are found tightly juxtaposed to ER subdomains that are competent for the production of COPII-coated transport carriers. In contrast to many unicellular systems, metazoan COPII carriers largely transit just a few hundred nanometers to the ERGIC, prior to COPI-dependent transport on to the cis-Golgi. The mechanisms underlying formation and maintenance of ERGIC membranes are poorly defined. However, recent evidence suggests an important role for Trk-fused gene (TFG) in regulating the integrity of the ER/ERGIC interface. Moreover, in the absence of cytoskeletal elements to scaffold tracks on which COPII carriers might move, TFG appears to promote anterograde cargo transport by locally tethering COPII carriers adjacent to ERGIC membranes. This action, regulated in part by the intrinsically disordered domain of TFG, provides sufficient time for COPII coat disassembly prior to heterotypic membrane fusion and cargo delivery to the ERGIC.

Keywords: Sar1; Sec12; Sec23; Tango1; liquid droplets; membrane trafficking; phase separation.

Figure 1.

Organization of the early secretory pathway in cultured human cells. Confocal imaging highlighting the unique distributions of COPII and the cis-Golgi in human cells. Scale bar, 5 μm.

Figure 2.

Morphology of COPII budding sites on the ER in metazoans. Intact C. elegans were high pressure frozen, freeze substituted and embedded in plastic prior to thin section electron microscopy analysis. Arrows highlight the ER and ERGIC membranes in the one-cell stage embryo, and negative membrane curvature associated with COPII budding sites is indicated with an arrowhead. Additional examples of COPII budding sites that are found in mammalian cells have been described previously.^[21] ENDO, endosome. Scale bar, 100 nm.

Figure 3.

Human Sar1B promotes membrane tubulation in vitro. Fluid supported bilayers with excess membrane reservoir (SUPER) templates^[59] were imaged using confocal optics in the presence or absence of purified human Sar1B (1 μM) bound to GTP. Scale bar, 2 μm.

Figure 4.

Model depicting COPII-mediated transport at the ER/ERGIC interface. Cartoon showing the anterograde movement of COPII-coated transport carriers between the ER and juxtaposed ERGIC membranes. The ongoing formation of cargo-laden carriers promotes TFG (green) recruitment and concentration, potentially leading to phase separation and local tethering until dissociation of all COPII components (yellow and red) and exposure of specific SNARE molecules, which mediated fusion with ERGIC membranes.