TANGO1 (transport and Golgi organization 1) interacts with CTAGE5 and COPII components Sec23/Sec24 and recruits ERGIC-53 (endoplasmic reticulum [ER]–Golgi intermediate compartment 53)–containing membranes to generate a mega-transport carrier for export of collagens and apolipoproteins from the ER. We now show that TANGO1, at the ER, assembles in a ring that encircles COPII components. The C-terminal, proline-rich domains of TANGO1 molecules in the ring are initially tilted onto COPII coats but appear to be pushed apart as the carrier grows. These findings lend support to our suggestion that growth of transport carriers for exporting bulky cargoes requires addition of membranes and not simply COPII-mediated accretion of a larger surface of ER. TANGO1 remains at the neck of the newly forming transport carrier, which grows in size by addition of ERGIC-53–containing membranes to generate a transport intermediate for the export of bulky collagens.

V. Malhotra is an Institució Catalana de Recerca i Estudis Avançats professor at the Centre for Genomic Regulation, and the work in his laboratory is funded by grants from the Ministerio de Economía, Industria y Competitividad Plan Nacional (ref. BFU2013-44188-P) and Consolider (CSD2009-00016). We acknowledge support of the Spanish Ministry of Economy and Competitiveness, through the Programmes “Centro de Excelencia Severo Ochoa 2013-2017” (SEV-2012-0208) and Maria de Maeztu Units of Excellence in R&D (MDM-2015-0502). The research leading to these results has received funding from the European Union Seventh Framework Program (FP7/2007-2013) under European Research Council grant agreement no. 268692 to V. Malhotra.