← Back

Module 2 · Vesicle Machinery

COPI & COPII Vesicles

Membrane budding in the early secretory pathway uses two dedicated coat-protein systems: COPII for ER-to-Golgi anterograde and COPI for intra-Golgi and Golgi-to-ER retrograde. Each uses a small GTPase (Sar1 or Arf1) to initiate budding and a protein coat to shape and select cargo. Together they account for nearly all traffic in and around the Golgi.

1. COPII: ER Exit

Schekman’s 1980 yeast-genetic screens for secretion mutants (secgenes) identified the machinery (Nobel 2013). COPII coat assembly:

  1. Sar1-GDP → Sar1-GTPexchanged by ER-resident Sec12 GEF. GTP binding exposes an amphipathic N-terminal helix that inserts into the cytoplasmic leaflet of ER exit sites.
  2. Sec23/Sec24 heterodimer recruited to Sar1-GTP. Sec24 carries the cargo-binding sites.
  3. Sec13/Sec31 outer cage assembles, polymerising to form the lattice that shapes the vesicle.
  4. Vesicle pinches off; Sar1-GTP hydrolysed by Sec23 GAP activity (Sec31 stimulates); coat disassembles.

Cargo selection: Sec24 has multiple independent binding pockets recognising cytoplasmic sorting signals (YxxM, DxE, di-acidic motifs). Different Sec24 isoforms (A/B/C/D in mammals) carry different cargo-binding preferences, producing specialised COPII vesicles for distinct cargo sets.

Large cargo problem revisited: COPII vesicles are ~60–80 nm; procollagen is > 300 nm. TANGO1 (Saito 2009) specifically packages procollagen into enlarged ER exit sites. Mutations in TANGO1 cause osteogenesis imperfecta — direct consequence of defective large-cargo secretion.

2. COPI: Retrograde and Intra-Golgi

COPI uses the Arf family small GTPase (Arf1 principally) and a seven-subunit protein coat called coatomer(α, β, β′, γ, δ, ε, ζ COP). Arf1-GTP inserts an amphipathic helix into the donor membrane; coatomer binds Arf1-GTP and polymerises to deform and pinch the vesicle. Arf1-GEFs: BIG1/2, GBF1 (BFA-sensitive). Arf1-GAP: ArfGAP1/2/3.

COPI cargo recognition: cytoplasmic motifs KKXX or KXKXX (Lys-rich C-terminal tails of type-I membrane proteins) bind the γ-subunit of coatomer and are retrieved from the Golgi back to the ER. The KDEL receptor (KDELR, ERD2) recognises the C-terminal KDEL motif of luminal ER residents (BiP, PDI, calreticulin); it binds cargo in the Golgi’s slightly acidic environment and releases it in the ER’s more neutral environment, shuttling back and forth via COPI vesicles.

COPI also carries intra-Golgi retrograde traffic: glycosyltransferases returning from trans to medial to cis as younger cisternae mature past them (Module 1).

3. Brefeldin A: A Classical Tool

Brefeldin A (BFA), a fungal metabolite, inhibits the Arf1-GEF GBF1 (and BIG1/2) by trapping them in a non-productive complex with Arf1-GDP. Consequence: no Arf1-GTP, no COPI assembly, retrograde traffic collapses, Golgi residents mix with the ER. BFA treatment dissolves the Golgi into the ER within minutes — a dramatic and reversible disruption that made BFA an indispensable experimental tool. The resolution of the BFA puzzle (Peyroche 1999, Mossessova 2003) pinned down the Arf1-GEF-GDP-GTP cycle at atomic resolution.

4. Vesicle Fusion: SNAREs, Rabs, Tethers

After budding, a vesicle must fuse with its target membrane. Three layers of specificity:

  • Rab GTPases (60+ in mammals) label membrane identity: Rab1 for ER-Golgi, Rab6 for intra-Golgi, Rab2 for ERGIC, Rab11 for recycling endosome, etc. A Rab in its GTP-bound state recruits effectors (tethers, motors, SNAREs).
  • Tethers: long coiled-coil proteins (golgins; p115; GARP complex; CATCHR family: Exocyst, COG, Dsl1, GARP) that catch incoming vesicles before SNAREs engage. They add specificity and can hold a vesicle ~100–200 nm from the target membrane.
  • SNAREs: the four-helix trans-SNARE bundle (v-SNARE on vesicle, three t-SNAREs on target) that drives fusion. Qa, Qb, Qc, R SNARE classification (Rössner & Jahn). Disassembled by NSF/α-SNAP after fusion for recycling.

Rothman shared the 2013 Nobel for the SNARE hypothesis (demonstrated biochemically with Weber 1998); Südhof for synaptic-vesicle fusion; Schekman for the vesicle-trafficking genetics.

5. Other Coated Vesicles

Beyond COPI/COPII: clathrin-coated vesicles(CCVs) handle post-Golgi endocytosis and some TGN-to-endosome traffic, using clathrin triskelia + adaptor complexes (AP-1, AP-2, AP-3, GGA family). Caveolae use caveolin lattices. Retromer (VPS26/29/35) handles endosome-to-Golgi retrograde recycling without obvious coat morphology. Each pathway has its own specific cargo-recognition signals and, accordingly, its own set of diseases.

← Module 1Module 3: Glycosylation →