TFG Promotes Organization of Transitional ER and Efficient Collagen Secretion

New insights into protein secretion: TANGO1 runs rings around the COPII coat

In this issue, Liu et al. (2017. J. Cell Biol. https://doi.org/10.1083/jcb.201611088) and Raote et al. (2017. J. Cell Biol. https://doi.org/10.1083/jcb.201608080) use super-resolution microscopy to visualize large COPII-coated endoplasmic reticulum (ER) export carriers. Rings of TANGO1 surround COPII, implicating TANGO1 in organizing ER exit sites and in regulating COPII coat dynamics and geometry.

TANGO1 assembles into rings around COPII coats at ER exit sites

TANGO1 interacts with COPII components to generate a transport carrier for export of large cargo from the endoplasmic reticulum. Raote et al. show that TANGO1 molecules assemble to form a closed ribbon structure that encircles COPII components.

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.

Tango1 spatially organizes ER exit sites to control ER export

Exit of secretory cargo from the endoplasmic reticulum (ER) takes place at specialized domains called ER exit sites (ERESs). In mammals, loss of TANGO1 and other MIA/cTAGE (melanoma inhibitory activity/cutaneous T cell lymphoma-associated antigen) family proteins prevents ER exit of large cargoes such as collagen. Here, we show that Drosophila melanogaster Tango1, the only MIA/cTAGE family member in fruit flies, is a critical organizer of the ERES-Golgi interface. Tango1 rings hold COPII (coat protein II) carriers and Golgi in close proximity at their center. Loss of Tango1, present at ERESs in all tissues, reduces ERES size and causes ERES-Golgi uncoupling, which impairs secretion of not only collagen, but also all other cargoes we examined. Further supporting an organizing role of Tango1, its overexpression creates more and larger ERESs. Our results suggest that spatial coordination of ERES, carrier, and Golgi elements through Tango1's multiple interactions increases secretory capacity in Drosophila and allows secretion of large cargo.

The calcium-binding protein ALG-2 promotes endoplasmic reticulum exit site localization and polymerization of Trk-fused gene (TFG) protein

Apoptosis-linked gene 2 (ALG-2), which is a gene product of PDCD6, is a 22-kDa Ca2+ -binding protein. Accumulating evidence points to a role for ALG-2 as a Ca2+ -responsive adaptor protein. On binding to Ca2+ , ALG-2 undergoes a conformational change that facilitates its interaction with various proteins. It also forms a homodimer and heterodimer with peflin, a paralog of ALG-2. However, the differences in cellular roles for the ALG-2 homodimer and ALG-2/peflin heterodimer are unclear. In the present study, we found that Trk-fused gene (TFG) protein interacted with the ALG-2 homodimer. Immunostaining analysis revealed that TFG and ALG-2 partially overlapped at endoplasmic reticulum exit sites (ERES), a platform for COPII-mediated protein transport from the endoplasmic reticulum. Time-lapse live-cell imaging demonstrated that both green fluorescent protein-fused TFG and mCherry-fused ALG-2 are recruited to ERES after thapsigargin treatment, which raises intracellular Ca2+ levels. Furthermore, overexpression of ALG-2 induced the accumulation of TFG at ERES. TFG has an ALG-2-binding motif and deletion of the motif decreased TFG binding to ALG-2 and shortened its half-life at ERES, suggesting a critical role for ALG-2 in retaining TFG at ERES. We also demonstrated, by in vitro cross-linking assays, that ALG-2 promoted the polymerization of TFG in a Ca2+ -dependent manner. Collectively, the results suggest that ALG-2 acts as a Ca2+ -sensitive adaptor to concentrate and polymerize TFG at ERES, supporting a potential role for ALG-2 in COPII-dependent trafficking from the endoplasmic reticulum.

Distinct isoform-specific complexes of TANGO1 cooperatively facilitate collagen secretion from the endoplasmic reticulum

The short isoform of TANGO1, termed TANGO1S, is necessary for collagen secretion from the ER independently of TANGO1L. TANGO1L and TANGO1S form individual complexes at ER exit sites and cooperatively participate in collagen export from the ER.

Collagens synthesized within the endoplasmic reticulum (ER) are too large to fit in conventional COPII-coated transport vesicles; thus their export from the ER requires specialized factors. TANGO1 (L) is an integral membrane protein that binds to collagen and the coatomer of vesicles and is necessary for collagen secretion from the ER. Here we characterized the short isoform of TANGO1 (TANGO1S), lacking the collagen-binding domain, and found that it was independently required for collagen export from the ER. Moreover, we found that each of the TANGO1 isoforms forms a stable protein complex with factors involved in collagen secretion: TANGO1L/cTAGE5/Sec12 (900 kDa) and TANGO1S/cTAGE5/Sec12 (700 kDa). Of interest, TANGO1S and TANGO1L seemed to be interchangeable in exporting collagen from the ER. Our results suggest that mammalian ER exit sites possess two different-sized membrane-bound macromolecular complexes that specifically function in large-cargo export from the ER.