Cisternal Maturation

1. The Classical Vesicular Model

Rothman’s reconstitution of intra-Golgi transport in the 1980s (Nobel 2013 shared with Schekman & Südhof) characterised the NSF/SNAP machinery of vesicle fusion. Electron micrographs showed vesicles between cisternae. The natural interpretation: cargo travels in vesicles from cis to trans, skipping across static cisternae.

Problems emerged as cargo got larger. Procollagen trimers form 300 nm rod-shaped aggregates in the cis-Golgi of fibroblasts; they cannot fit in the 60–80 nm vesicles visible in EM. Yet they transit the Golgi. This was the “large-cargo problem” and it was incompatible with a pure vesicular model.

2. The Cisternal-Maturation Alternative

Glick & Malhotra (1998) and others argued that the cisternae themselves are transient, and cargo rides passively as the cisterna matures:

1. A new cisterna is formed at the cis face by COPII-vesicle fusion carrying ER exit material.
2. The cisterna acquires resident enzymes (cis-Golgi enzymes) delivered by retrograde COPI vesicles from older cisternae.
3. As the cisterna progresses, it discards its early-Golgi enzymes (returned retrograde to younger cisternae by COPI), acquires medial enzymes (again by retrograde COPI from older cisternae), then trans-Golgi enzymes.
4. Eventually the cisterna reaches the TGN, is broken into transport vesicles, and its cargo dispersed.

Under this model, COPI vesicles carry enzymes, not cargo, in the retrograde direction. The forward (cis-to-trans) flux is the cisterna itself. Cargo size is unconstrained — whatever fits in the cisterna travels.

3. The Yeast Live-Imaging Proof

Two 2006 papers — Matsuura-Tokita et al. and Losev et al. — used live-cell imaging in budding yeast, where the Golgi is not a stack but dispersed single cisternae. They fluorescently labelled cis-Golgi enzymes in one colour and trans-Golgi enzymes in another. Tracking individual cisternae showed them changing colour over time: a cisterna that was green at t = 0 became red by t = 3 minutes. This is direct evidence for cisternal maturation: the compartment itself is dynamic.

Mammalian evidence followed from FRAP on GFP-tagged residents, from large-cargo tracking, and from mathematical modelling of the compatibility of enzyme gradients with maturation. The modern consensus: maturation is the dominant mechanism, with vesicles playing a specific role in retrograde transport of residentsrather than anterograde cargo carriage.

4. Maintaining the Enzyme Gradient

If cisternae mature, how do glycosyltransferases stay concentrated where they should be (cis enzymes at cis, trans enzymes at trans)? Two mechanisms:

• Retrograde transport: as an enzyme moves forward with its cisterna, it is repeatedly extracted by retrograde COPI vesicles and delivered back to a younger cisterna. Net effect: enzymes sit in a dynamic steady state at their preferred location.
• Transmembrane-domain length sorting(Munro 1995): trans-Golgi enzymes have shorter TM domains than PM proteins; this biases them into cholesterol-poor, thinner-bilayer regions of the Golgi. Lipid-domain partitioning thus contributes to sub-Golgi targeting.

Glycosyltransferase sorting signals (COPI-binding motifs on cytoplasmic tails) direct retrograde recognition. Loss of these signals (deletion of the cytoplasmic tail) causes the enzyme to mature forward indefinitely and end up at the plasma membrane.

5. The Current Synthesis

The maturation model is the current consensus, but it is not pure. Small cargo may travel anterograde in vesicles between cisternae (the “rim progression” of Glick), complementing the bulk maturation flux. Very fast-secreting proteins (e.g., VSV-G under temperature shift) show kinetics that suggest direct cis-to-trans vesicle carriage. The Golgi is probably a hybrid — maturation dominates for large cargo and resident enzymes, vesicular transport supplements for small cargo. The debate’s real legacy is having forced the field to develop quantitative tools for measuring intra-Golgi traffic.