COPI-coated ER-to-Golgi transport complexes segregate from COPII in close proximity to ER exit sites

TLDR

Transport of proteins between the ER and Golgi is mediated by COPI and COPII coat complexes, with COPII selecting cargo and forming vesicles at the ER and COPI recycling transport machinery from post‑ER membranes. The study aimed to test this transport model in vivo and determine the temporal sequence of COPI and COPII action during ER‑to‑Golgi transport. Time‑lapse microscopy of living cells was used to simultaneously visualize COPII, COPI, and GFP‑tagged secretory markers. In vivo imaging showed COPII confined to ER membranes, secretory cargo segregating and moving to the Golgi without COPII, and COPI complexes forming adjacent to COPII sites, then segregating and traveling directionally to the Golgi, confirming a two‑step sequential COPII‑then‑COPI transport. Movies are available online at http://www.biologists.com/JCS/movies/jcs1368.html.

Abstract

ABSTRACT Transport of proteins between the endoplasmic reticulum and Golgi apparatus is mediated by two distinct membrane coat complexes, COPI and COPII. Genetic, biochemical and morphological data have accumulated into a model which suggests a sequential mode of action with COPII mediating the selection of cargo and formation of transport vesicles at the ER membrane for ER-to-Golgi transport and COPI mediating recycling of the transport machinery from post-ER membranes. To test this transport model directly in vivo, and to study the precise temporal sequence of COPI and COPII action in ER-to-Golgi transport, we have used time lapse microscopy of living cells to visualise simultaneously the dynamics of COPII and COPI, as well as COPII and GFP tagged secretory markers in living cells. The majority of COPII labelling appears tightly associated with ER membranes that move only within a limited area (less than 2 μm). Secretory cargo segregates from these sites and is then transported to the Golgi apparatus without any apparent association with COPII. COPI-coated transport complexes are seen to form adjacent to the COPII sites on the ER before segregating and moving directionally towards the Golgi apparatus. COPII is not present on these transport complexes and remains associated with the ER. These data demonstrate for the first time directly in vivo that ER-to-Golgi transport is organised in two steps characterised by a sequential mode of action of COPII and COPI. Movies available on-line: http://www.biologists.com/JCS/movies/jcs1368.html