TAN GO1 assembles into rings around COP II coats at ER exit sites

Abstract

TAN GO1 (transport and Golgi organization 1) interacts with CTA GE5 and COP II components Sec23/Sec24 and recruits ERG IC-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 TAN GO1, at the ER, assembles in a ring that encircles COP II components. The C-terminal, proline-rich domains of TAN GO1 molecules in the ring are initially tilted onto COP II 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 COP II-mediated accretion of a larger surface of ER. TAN GO1 remains at the neck of the newly forming transport carrier, which grows in size by addition of ERG IC-53-containing membranes to generate a transport intermediate for the export of bulky collagens.