Sequential coupling between COPII and COPI vesicle coats in endoplasmic reticulum to Golgi transport.

TLDR

COPI and COPII are vesicle coat complexes whose assembly is regulated by the ARF1 and Sar1 GTPases, respectively. The study proposes that sequential coupling between COPII and COPI coats is essential to coordinate bi‑directional vesicular traffic between the ER and pre‑Golgi intermediates during protein transport to the Golgi. Stage‑specific in vitro transport assays with GDP‑ and GTP‑restricted Sar1 and ARF1 were used to synchronize cargo movement and control coat recruitment. COPII alone drives ER export and initiates pre‑Golgi vesicle assembly, while COPI is recruited later to pre‑Golgi intermediates to segregate anterograde VSV‑G from retrograde p58, demonstrating independent and sequential recruitment of the two coats.

Abstract

COPI and COPII are vesicle coat complexes whose assembly is regulated by the ARF1 and Sar1 GTPases, respectively. We show that COPI and COPII coat complexes are recruited separately and independently to ER (COPII), pre-Golgi (COPI, COPII), and Golgi (COPI) membranes of mammalian cells. To address their individual roles in ER to Golgi transport, we used stage specific in vitro transport assays to synchronize movement of cargo to and from pre-Golgi intermediates, and GDP- and GTP-restricted forms of Sar1 and ARF1 proteins to control coat recruitment. We find that COPII is solely responsible for export from the ER, is lost rapidly following vesicle budding and mediates a vesicular step required for the build-up of pre-Golgi intermediates composed of clusters of vesicles and small tubular elements. COPI is recruited onto pre-Golgi intermediates where it initiates segregation of the anterograde transported protein vesicular stomatitis virus glycoprotein (VSV-G) from the retrograde transported protein p58, a protein which actively recycles between the ER and pre-Golgi intermediates. We propose that sequential coupling between COPII and COPI coats is essential to coordinate and direct bi-directional vesicular traffic between the ER and pre-Golgi intermediates involved in transport of protein to the Golgi complex.