Past exam of the zoology course of the University of Cambridge / 2025 / Paper zm6 / 6

Coat-associated adaptors couple three tasks: recognizing the correct membrane, selecting cargo, and assembling a carrier. Each interaction can be modest on its own; coincidence detection creates a selective assembly site. A sorting motif on a cargo's cytoplasmic tail can contact an adaptor directly, whereas soluble luminal cargo requires a transmembrane receptor to expose an adaptor-binding signal to the cytoplasm. Thus cargo recognition respects membrane orientation.

In clathrin-mediated endocytosis, the AP2 adaptor complex recognizes plasma-membrane determinants and cargo motifs while recruiting the clathrin scaffold. Other clathrin adaptors operate at internal membranes, so clathrin alone does not specify an itinerary. AP1 adaptor complex-associated carriers, for example, rely on a different local molecular context. Cargo, lipid and small-GTPase interactions collectively distinguish productive assembly from accidental coat binding.

COPII illustrates the same logic at endoplasmic reticulum exit sites. Membrane-associated Sar1 GTPase recruits the inner Sec23-Sec24 complex layer, in which Sec24 recognizes cargo and cargo receptors; Sec13-Sec31 complex builds an outer scaffold. COPI uses a different coat assembly, recruited by ARF-family GTPases, and participates in retrieval toward the endoplasmic reticulum and traffic within the Golgi apparatus. These systems need not have identical subunits to share the architecture of membrane recruitment, cargo selection and scaffold assembly.

An especially useful mechanistic test is biochemical reconstitution. Purified COPII coat components and defined liposomes were sufficient to produce coated vesicles, demonstrating that assembly itself can deform membranes rather than merely decorate carriers made by a wholly independent mechanism.^[ref] Sufficiency in this minimal system does not imply that every regulator and cargo in a living cell is dispensable.

Coat polymerization, membrane-inserting regions and accessory factors contribute to curvature, while fission machinery and details of neck geometry differ among pathways. The nucleotide cycles of Sar1 GTPase and ARF-family GTPases regulate coat recruitment and turnover; they should not be portrayed as identical engines executing every membrane-cutting step. After budding, substantial uncoating permits subsequent targeting and fusion. Destination specificity then depends on additional factors, including tethering machinery, Rab-family GTPases and SNAREs; a coat is not a complete postal address by itself.

To test the common design, mutate a cargo motif, its adaptor-binding site and the membrane-recruitment determinant separately. Measure recruitment, cargo concentration and productive transport as separate outputs, and rescue with the corresponding wild-type component. A cargo-selective defect without complete loss of carrier formation is particularly informative because it separates selection from the physical formation of a carrier.

Ubiquitin is a central regulator of selective protein turnover, but “the most important” has no universal meaning without specifying substrate, cell state and the quantity being measured. Selective destruction of a short-lived regulator, bulk recycling during starvation, and removal of a damaged mitochondrion are different problems. The ubiquitin system is extensive, yet it neither encompasses every degradation pathway nor makes all ubiquitination a destruction signal.

In the canonical pathway, an E1 enzyme uses ATP to activate ubiquitin, an E2 enzyme carries it, and an E3 ligase supplies much of the substrate specificity. Repeated conjugation can build chains on a substrate, while deubiquitinating enzymes edit or remove those modifications. Linkage and chain architecture influence recognition; a K48-linked chain is a common proteasomal signal, but “polyubiquitinated” does not specify a single outcome. Some ubiquitinations regulate location or signalling instead of degradation.

The proteasome recognizes suitable substrates, removes or recycles ubiquitin, unfolds accessible protein regions using ATP-dependent machinery, and threads the polypeptide into a proteolytic core. Substrate tagging is therefore one control point among several: accessibility, engagement and unfolding matter as well. This permits rapid removal of regulatory proteins and disposal of misfolded soluble proteins without digesting an entire organelle.

A particularly clear exception is ornithine decarboxylase. In a cell-free system, proteasome depletion blocked its ATP- and antizyme-dependent degradation; purified 26S proteasomes supported degradation without ubiquitin. This provides direct biochemical evidence that proteasomal destruction need not always require substrate ubiquitination.^[ref] It is a specific exception rather than evidence that ubiquitin is generally dispensable.

Lysosomes provide another major route. Autophagy can sequester cytoplasm, aggregates and organelles in membrane carriers for lysosomal degradation. Some selective autophagy receptors recognize ubiquitinated cargo, so this route is partly coordinated with ubiquitin; bulk starvation-induced autophagy and other cargo-recruitment mechanisms show why it is not equivalent to proteasomal tagging. A yeast genetic screen used failure to accumulate autophagic bodies and failure of starvation-induced vacuolar protein turnover to identify autophagy-defective mutants. The experiment established a genetically separable membrane-delivery programme essential to this recycling response.^[ref]

Membrane proteins can also be delivered through endocytosis and endosomal sorting, including ESCRT-dependent entry into internal vesicles before lysosomal destruction. Here ubiquitin often helps specify sorting, while receptor internalization, membrane traffic and lysosomal activity supply additional control points. Dedicated organellar proteases add further pathways.

Experimentally, compare proteasome inhibition, lysosomal inhibition and perturbation of a defined E3 ligase or autophagy factor while following a labelled substrate over time. A steady-state increase is insufficient: it can result from increased synthesis rather than reduced degradation. Measure turnover and pathway flux, check inhibitor toxicity and use genetic rescue. The defensible conclusion is that ubiquitin is a major, versatile selectivity signal whose importance is context-dependent; complete protein clearance depends on several regulated degradation systems.