COPII and COPI mediate the formation of membrane vesicles translocating in opposite directions within the secretory pathway. Live-cell and electron microscopy revealed a novel mode of function for COPII during cargo export from the ER. COPII is recruited to membranes defining the boundary between the ER and ER exit sites, facilitating selective cargo concentration. Using direct observation of living cells, we monitored cargo selection processes, accumulation, and fission of COPII-free ERES membranes. CRISPR/Cas12a tagging, the RUSH system, and pharmaceutical and genetic perturbations of ER-Golgi transport demonstrated that the COPII coat remains bound to the ER–ERES boundary during protein export. Manipulation of the cargo-binding domain in COPII Sec24B prohibits cargo accumulation in ERES. These findings suggest a role for COPII in selecting and concentrating exported cargo rather than coating Golgi-bound carriers. These findings transform our understanding of coat proteins’ role in ER-to-Golgi transport.

Lipid droplets (LDs) comprise a triglyceride core surrounded by a lipid monolayer enriched with proteins, many of which function in LD homeostasis. How proteins are targeted to the growing LD is still unclear. Rab1b, a GTPase regulating secretory transport, was recently associated with targeting proteins to LDs in a Drosophila RNAi screen. LD formation was prevented in human hepatoma cells overexpressing dominant-negative Rab1b. We thus hypothesized that Rab1b recruits lipid-synthesizing enzymes, facilitating LD growth. Here, FRET between diacylglycerol acyltransferase 2 (DGAT2) and Rab1b and activity mutants of the latter demonstrated that Rab1b promotes DGAT2 ER to the LD surface redistribution. Last, alterations in LD metabolism and DGAT2 redistribution, consistent with Rab1b activity, were caused by mutations in the Rab1b–GTPase activating protein TBC1D20 in Warburg Micro syndrome (WARBM) model mice fibroblasts. These data contribute to our understanding of the mechanism of Rab1b in LD homeostasis and WARBM, a devastating autosomal-recessive disorder caused by mutations in TBC1D20.

Human colonic organoids derived from adult tissue biopsies are based on the ability of isolated somatic epithelial stem cells to reconstitute the structure and function of the colon, offering new opportunities for studying the biology of the large intestine in both health and disease. These colonoids may also function as efficient platforms for drug screening and discovery. Here, we describe the establishment of human colonic organoids derived from healthy, and adenomatous polyp tissues. We then demonstrate that organoids grown from adenomas of familial adenomatous polyposis (FAP) patients harboring nonsense mutations in the tumor suppressor gene adenomatous polyposis coli (APC), can be used to establish a personalized therapeutic strategy which relies on nonsense mutation readthrough therapy.