Past exam of the biochemistry course of the University of Cambridge / 2025 / Paper 5 / Section 1 / 1

Partial proteolysis is one plausible explanation: intact SNAP25 and a slightly shortened form would migrate as two nearby bands in SDS-PAGE. Heterogeneous processing, a retained affinity tag, or an anomalously migrating species are other possibilities. The gel alone does not identify the chemical difference. Because the proteins were expressed in Escherichia coli, a mammalian post-translational modification should not simply be assumed. A Western blot establishing that both bands contain SNAP25, followed by mass spectrometry, could distinguish these explanations.

The liposomes could be separated from free proteins by density-gradient flotation: lipid-containing particles float to a different position from soluble protein. Size-exclusion chromatography is another possibility if it resolves the large vesicles from the unincorporated proteins. Simple sedimentation can also collect sufficiently large vesicles, but protein aggregates may sediment too. The important control is to show that the recovered SNARE proteins associate with the lipid bilayer, rather than merely copurifying with it.

The recovered vesicle fractions contain bands at the expected positions for the supplied SNARE proteins. The v-SNARE preparation contains its principal band, and the t-SNARE preparation contains both t-SNARE and the SNAP25 doublet. Thus protein-containing vesicle fractions have been recovered, consistent with successful reconstitution.

The SDS-PAGE experiment does not establish the orientation of the proteins, their proportion facing outward, the stoichiometry in an individual vesicle, or membrane fusion. Nor does a stained band prove that all of its protein is correctly folded and active. Those are separate functional and membrane topology questions.

After warming, more labelled v-SNARE remains full length: lane 2 is approximately 80–85% of the reference signal, compared with about 40–45% in lane 1. Formation of a protected SNARE complex provides a consistent explanation: the susceptible part of v-SNARE is less accessible after complex assembly.

Protection is not itself a unique readout of membrane fusion. Some protein may face the vesicle lumen, and SNARE complexes can protect protein without proving that the two aqueous compartments have merged. The inference depends on comparing these alternatives with the detergent and partner-free controls.

In lane 3, detergent removes the lipid bilayer barrier while both partner preparations are present. Their binding regions can then encounter one another even if they originally faced inward or were on poorly apposed lipid bilayers. The strong remaining full-length signal is consistent with extensive formation of protected SNARE complexes.

Thus detergent does not prevent partner association under the displayed conditions; it removes the topological restriction on that association. This lane is the normalization reference, not a direct measurement that 100% of all initially supplied protein is active. The weak signal in the detergent-treated partner-free control also argues against detergent simply preventing all cleavage.

Lane 4 combines detergent with the absence of the t-SNARE preparation. The lipid bilayer no longer protects inward-facing v-SNARE, and there is no t-SNARE partner to provide complex-dependent protection. Consequently little full-length labelled protein remains. This distinguishes membrane-dependent protection from partner-dependent protection in a protease protection assay. It also helps interpret lane 3: its strong signal requires the partner rather than lipid bilayer enclosure alone.

Use a membrane topology measurement on v-SNARE vesicles alone, so that partner binding cannot mask an outward-facing domain. A membrane-impermeant probe directed against the binding-domain region would label only the exposed population while vesicles remain intact. Compare that signal with total accessible protein after lipid bilayer disruption, using a trapped-lumen marker to check that the intact vesicles exclude the probe.

With comparable detection efficiencies and background subtraction,An analogous protease protection assay compares external accessibility with total accessibility. In either approach, lipid bilayer leakage and steric masking must be distinguished from genuinely inward-facing protein.

NBD and rhodamine are initially close together in the labelled vesicle lipid bilayer, allowing Förster resonance energy transfer and suppressing donor fluorescence. Mixing labelled lipid bilayer with unlabelled lipid bilayer dilutes the dyes, decreases energy transfer and increases the NBD signal.

The rise with the complementary SNARE proteins, but little rise with protein-free vesicles, therefore supports SNARE-dependent lipid mixing. It is consistent with membrane fusion, but does not by itself distinguish complete fusion from hemifusion or other causes of dye dilution. A separate content mixing assay with a leakage control would test merger of the aqueous compartments.

There is no evidence for substantial lipid mixing during the cold preincubation. The trace begins near its low-fluorescence baseline when warming starts; a substantial pre-existing dilution of the dyes would already have raised that baseline. Nevertheless, vesicle docking or trans-SNARE complex formation can accumulate while the later membrane-rearrangement step remains slow. Absence of dequenching does not mean that the vesicles have had no interaction.

Cold preincubation can accumulate docked, fusion-ready intermediates. On warming, that population proceeds promptly to lipid mixing, whereas newly combined vesicles must first encounter, dock and assemble productive SNARE complexes. This explains the faster initial rise and larger signal at the displayed endpoint without requiring a different total amount of fluorescent lipid.

The original reconstitution experiments likewise distinguish docking from subsequent membrane fusion.^[ref]

Triton disperses the lipid bilayer into detergent-containing particles, separating the fluorescent lipids enough to remove most fluorescence quenching. Both traces approach the maximum-dilution reference even though their earlier lipid-mixing histories differed. The final fluorescence jump is detergent-induced dequenching, not an additional burst of physiological membrane fusion. It provides a useful normalization endpoint for the available dye signal.

The N-terminal 1–29 fragment and the 1–15 fragment inhibit invadopodium formation. Their values are around one fifth of the control, and gelatin degradation is strongly reduced too. Full-length t-SNARE, empty vector and the 15–29 fragment remain near control values. The images agree: the inhibitory fragments leave much more intact gelatin beneath the cells. The corresponding published syntaxin-4 study links this phenotype to impaired cargo trafficking.^[ref]

The 1–15 fragment has a slightly lower mean degradation signal and perhaps lower invadopodium formation than the longer N-terminal fragment. It may be the stronger inhibitor, but the displayed comparisons do not establish that difference. The asterisks compare groups with control, not necessarily the two active fragments with one another. Their error bars overlap, and overlap alone is neither a definitive test of equality nor a calibrated significance test.

A direct comparison needs replicate-level data and a suitable statistical hypothesis test, with an effect size and confidence interval. Comparable fragment abundance, cell viability and cell number are also needed before interpreting a phenotypic difference as greater molecular potency.

Three complementary readouts would separate local matrix degradation from invasive and metastatic behaviour:

Empty-vector and inactive-fragment controls are needed throughout. Neither reduced gelatin digestion alone nor fewer cells after treatment establishes a specific effect on metastasis.

The fragment might sequester another factor required for vesicle trafficking or actin organization, rather than acting through Munc18. For example, binding a different SNARE-associated protein could lower the concentration available for productive endogenous complexes. This is a possible dominant-negative effect, not evidence that the isolated N terminus contains the entire SNARE interaction surface.

A less specific alternative is cellular stress or reduced viability. Those effects could suppress both invadopodium formation and matrix degradation without selectively inhibiting the proposed trafficking pathway.

For the sequestration hypothesis, ask whether the fragment associates with the proposed alternative partner by co-immunoprecipitation, then compare endogenous complex assembly and cargo localization. An association should be evaluated against empty-vector and inactive-fragment controls, and an independent interaction readout would help distinguish direct binding from membership of a larger complex.

A useful functional test is whether restoring the limiting partner rescues trafficking and invadopodium formation without changing fragment expression or viability. Rescue would support the hypothesis, although overexpression can itself alter trafficking. Binding evidence, the endogenous phenotype and rescue should therefore agree; any one alone is insufficient.

The S61/E62 and S75/Q76 pairs are especially important for the extra stimulation attributable to Munc18. Their replacement leaves basal lipid mixing broadly present but makes the red and blue traces much closer together. In contrast, replacement of R47/V48 or D57/Q58 retains strong stimulation. The S75/E78/T79 replacement retains a smaller enhancement, suggesting a partial contribution under these conditions.

Compare the Munc18-dependent increment with each construct's own baseline, rather than comparing red endpoints alone. These results identify functionally important regions; they do not prove that every altered side chain directly contacts Munc18. The original reconstitution experiments also support cognate-SNARE recognition.^[ref]

V3 is a plausible responsive partner; V8 is less likely to show the same stimulation. V3 conserves the aligned S/E and S/Q pairs implicated by the V2 comparisons, whereas V8 differs at those positions. This supports a prediction of conserved recognition, not a conclusion that sequence identity guarantees the same kinetic response.

Other residues, protein folding, lipid bilayer orientation and partner compatibility matter too. In particular, comparing the aligned motifs is more informative here than comparing only the variable N-terminal tails. The prediction should be checked by equivalent functional comparisons, rather than inferred from the family names alone.