Audhya Lab
COPII vesicle biogenesis and secretion
research_tfg_on_the_er
The trafficking of most secretory cargoes, including proteins and lipids, begins with their export from the endoplasmic reticulum (ER). In metazoans, cargoes are packaged into vesicles that emerge at defined sites on the ER and ultimately fuse with the ER-Golgi intermediate compartment (ERGIC). This process relies on the efficient recruitment of a set of soluble factors known as the COPII coat, which is composed of two multimeric protein complexes (Sec23/24 and Sec13/31).

Although COPII coated vesicle formation has been reconstituted with purified coat proteins on synthetic membranes, regulators of COPII assembly in vivo remain largely unexplored. Of the known COPII interacting proteins, two membrane-associated proteins, the small GTPase Sar1 and the putative scaffolding protein Sec16, are among the best characterized. In its GTP bound state, an amphipathic α-helix within Sar1 is exposed and inserts into the lipid bilayer to induce membrane curvature. Additionally, Sar1-GTP recruits the Sec23/24 complex, which forms an adaptor layer for Sec13/31 lattice assembly, completing the COPII coat. Sec23 also functions as a Sar1 GAP, which is ultimately stimulated by Sec31, leading to coat disassembly following vesicle budding.

In contrast to Sar1, which has only been shown to associate directly with Sec23, Sec16 interacts with multiple components of the COPII coat, potentially serving as a scaffold for their recruitment. Furthermore, Sec16 has been postulated to stabilize the COPII coat to prevent premature disassembly following activation of the Sec23 GAP. In vitro, Sec16 is dispensable for the generation of COPII coated vesicles. However, it can stimulate the function of COPII proteins, suggesting an important regulatory function. Consistent with this idea, multiple studies using a variety of model systems have demonstrated that Sec16 is essential for the exit of secretory proteins from the ER.

In addition to Sar1 and Sec16, few other proteins have been implicated in the regulation of COPII recruitment. Considering the necessity for controlling secretory flux during cell differentiation and development, additional factors that govern this process likely exist, and some may function via direct regulation of Sar1 or Sec16. To identify new regulators of ER export, we conduct biochemical and genetics screens to identify new components that control COPII-mediated vesicle biogenesis, vesicle budding, and vesicle fusion with post-ER compartments.