COPII: ER to Golgi Transport

Core Concept

COPII is the coat that buds vesicles from the endoplasmic reticulum and carries them forward (anterograde) to the Golgi — the first and mandatory step of the secretory pathway. COPII assembly is nucleated by the small GTPase Sar1. When Sar1 binds GTP at ER exit sites, it inserts into the membrane and recruits two coat layers: the inner Sec23/Sec24 heterodimer (which binds cargo and Sar1) and the outer Sec13/Sec31 cage (which deforms the membrane into a bud). Cargo is selected by ER exit signals (e.g., di-acidic DXE motifs) recognized by Sec24, while ER-resident proteins lacking such signals are left behind. The result is a cargo-rich vesicle that delivers its contents to the ER-Golgi intermediate compartment (ERGIC) and then the Golgi.

Key Components

• Sar1: a small GTPase; its GTP-bound, membrane-inserted form nucleates the coat.
• Sec23/Sec24: inner coat complex; Sec23 is the GAP for Sar1; Sec24 binds cargo sorting signals.
• Sec13/Sec31: outer coat complex that self-assembles into a curved cage.
• Sec12: the ER-resident GEF that loads Sar1 with GTP.
• ER exit signals: cytoplasmic sorting motifs such as the di-acidic DXE (e.g., in VSV-G) or di-hydrophobic motifs.
• ER exit sites (ERES): specialized ER subdomains where COPII vesicles form.
• ERGIC (VTCs): vesicular-tubular clusters that ferry COPII cargo onward to the cis-Golgi.

Mechanism / How It Works

1. Activation. The ER-membrane GEF Sec12 exchanges GDP for GTP on cytosolic Sar1, exposing an amphipathic N-terminal helix that embeds in the ER membrane.
2. Inner coat. Membrane-bound Sar1-GTP recruits Sec23/Sec24; Sec24 binds cargo proteins that display ER exit signals (and binds transmembrane receptors that capture lumenal cargo).
3. Outer coat. Sec13/Sec31 polymerizes over Sec23/Sec24 into a cage whose curvature deforms the membrane into a bud, concentrating selected cargo.
4. Scission and uncoating. The cage closes and the vesicle pinches off. Sec23's GAP activity (stimulated by Sec13/31) promotes Sar1 GTP hydrolysis; Sar1-GDP detaches, destabilizing the coat, which disassembles to expose Rab and SNARE proteins.
5. Delivery. COPII vesicles fuse with each other and with the ERGIC, and cargo moves onward through the Golgi.

Energy and Directionality

COPII budding is a GTP-driven cycle. Sar1 uses GTP to bind the membrane and recruit the coat, and its subsequent GTP hydrolysis (accelerated by Sec23/Sec13-31) drives coat disassembly after scission — coupling the vesicle's formation and release to the nucleotide cycle. Directionality (ER → Golgi only) is ensured because Sec12 (the Sar1 GEF) is confined to the ER membrane; Sar1 cannot be activated on the Golgi, so COPII can only form at ER exit sites. No ATP or membrane potential is required for budding itself.

Experimental Evidence / Technique

COPII was identified through yeast secretory (sec) mutants: temperature-sensitive mutations in SEC12, SEC13, SEC23, SEC24, SEC31 block ER-to-Golgi transport, causing ER accumulation. The Nobel-recognized in vitro reconstitution by Randy Schekman's group showed that purified Sar1, Sec23/24, Sec13/31, and GTP are sufficient to form COPII vesicles from liposomes. Cryo-electron microscopy solved the Sec13/31 cage structure, and cargo-binding studies identified the DXE exit signal recognized by Sec24.

How it works

1. Activation. The ER-membrane GEF Sec12 exchanges GDP for GTP on cytosolic Sar1, exposing an amphipathic N-terminal helix that embeds in the ER membrane.
2. Inner coat. Membrane-bound Sar1-GTP recruits Sec23/Sec24; Sec24 binds cargo proteins that display ER exit signals (and binds transmembrane receptors that capture lumenal cargo).
3. Outer coat. Sec13/Sec31 polymerizes over Sec23/Sec24 into a cage whose curvature deforms the membrane into a bud, concentrating selected cargo.
4. Scission and uncoating. The cage closes and the vesicle pinches off. Sec23's GAP activity (stimulated by Sec13/31) promotes Sar1 GTP hydrolysis; Sar1-GDP detaches, destabilizing the coat, which disassembles to expose Rab and SNARE proteins.
5. Delivery. COPII vesicles fuse with each other and with the ERGIC, and cargo moves onward through the Golgi.

Common confusions

• "COPII moves Golgi → ER." — That is COPI (retrograde). COPII is ER → Golgi (anterograde).
• "Sar1 is activated by Ran or Rab." — Sar1 is a distinct small GTPase activated by the ER GEF Sec12.
• "COPII buds from the Golgi." — COPII buds only from ER exit sites; the Golgi uses COPI and clathrin.
• "Cargo is random; residents are retrieved later." — Cargo is actively selected by Sec24 binding exit signals, though some ER residents also leak out and are retrieved by COPI.
• "Budding needs dynamin." — Dynamin is for clathrin (and some other) scission; COPII scission is driven by the coat cage + Sar1 cycle, not dynamin.

Quick review

• COPII = anterograde ER → Golgi coat.
• Sar1-GTP (Sec12 GEF) → Sec23/24 → Sec13/31 cage → bud → scission → uncoating.
• Cargo selection via DXE exit signals bound by Sec24.
• GTP powers assembly/disassembly; Sec12 localization enforces directionality.
• Yeast sec mutants + in vitro reconstitution defined COPII.
• Disease: SEC23 mutations → bleeding disorder + craniofacial syndrome.