Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 1 / Section D

Gating controls when an ion pathway is available; selectivity controls which ions can use it. An open ion channel permits passive movement down an electrochemical gradient. Opening does not itself specify current direction or guarantee a net current at the reversal potential. At a simple level, a population current can be written $I=N{P}_o\gamma (V-E_{\mathrm{rev}})$, separating channel number, open probability, single-channel conductance and driving force.

In a typical voltage-gated potassium channel, four subunits each contain six transmembrane helices. The S1–S4 region forms a voltage-sensing domain; S4 carries regularly spaced positive charges. A change in membrane potential alters the relative free energies of voltage-sensor conformations. Sensor rearrangement is coupled to opening of the S5–S6 pore region, including its intracellular activation gate. The charges do not have to traverse the entire membrane as free ions: protein conformational changes redistribute them through the electric field. A simple two-state approximation giveswhere $z_g$ is an effective gating charge and the sign convention makes depolarization favour opening for $z_g>0$. Real channels usually have multiple closed states and more complex kinetics.

voltage-gated sodium channels have four homologous domains in one polypeptide, with related voltage sensors and a central pore. Activation is followed by inactivation, which is a distinct process from the activation gate closing when the membrane repolarizes. Calcium channels share a related architecture. Voltage clamp can separate activation and inactivation kinetics, while gating-current measurements and voltage-sensor mutations test coupling between charge movement and pore opening. A mutation that abolishes current must be checked for expression and membrane delivery before concluding that it identifies the gate.

For potassium selectivity, the narrow selectivity filter contains backbone carbonyl oxygens arranged to coordinate largely dehydrated potassium ions. Binding and permeation balance the energetic cost of shedding water against favourable coordination within the filter. Sodium is smaller, but does not receive the same stabilization in the potassium-selective environment, so selectivity is not explained by a rigid hole that rejects all larger ions and admits all smaller ones. The KcsA structure established a concrete molecular basis for this principle.^[ref] KcsA itself is not the canonical S4 voltage-gated channel: its pore illustrates a conserved selectivity mechanism, not the voltage sensor just described. Voltage-gated sodium channels use a different filter chemistry; eukaryotic calcium-channel filters favour divalent calcium through an acidic coordination environment.

A ligand-gated ion channel instead couples ligand occupancy to changes in pore-opening probability. The nicotinic acetylcholine receptor is a pentamer with extracellular ligand-binding interfaces and a pore lined by M2 helices. Acetylcholine binding favours an opening-associated conformational change transmitted through the extracellular-to-transmembrane interface. Its pore conducts cations, including sodium and potassium, rather than selectively conducting only the transmitter or only sodium. Negatively charged regions and pore geometry favour cation over anion permeation. The ligand-binding site and the ion-selective pathway are spatially distinct, so the chemical identity of the bound transmitter does not define the permeant ion.

Rapid agonist exposure followed by freezing of Torpedo receptor membranes captured an opening-associated conformation, experimentally connecting ligand application to a receptor structural state.^[ref]

Other ligand-gated channels illustrate the independence of these properties: GABA-A receptors primarily support anion conductance, while glutamate-receptor channels support cation conductance with subtype-dependent calcium permeability. A sustained agonist can produce channel desensitization, in which an occupied receptor becomes nonconducting; occupancy is therefore not a permanently open instruction. Intracellular ligands can gate other channels, including ATP-dependent regulation, so “ligand-gated” need not imply an extracellular neurotransmitter.

The decisive tests separate binding, gating and permeation. Patch clamp resolves individual opening and closing events; rapid ligand application measures activation and desensitization; ion substitution and reversal-potential measurements constrain relative permeability. Changing a filter residue while retaining agonist binding and surface expression can implicate selectivity, while changing binding-site affinity without changing unitary conductance suggests a different role. Appropriate controls include leakage currents, solution junction potentials, channel abundance and ionic activities. A structure supplies candidate mechanisms; state-dependent electrophysiology establishes what those mechanisms do.

Frog oocytes and egg extracts turned cell-cycle regulation into a transferable biochemical activity that could be purified, removed, restored and followed through time. Xenopus laevis provides large oocytes and abundant eggs containing stockpiled proteins and maternal messenger RNA. Early embryonic cycles can alternate rapidly between DNA replication and mitosis with little or no intervening growth phases. This makes extracts powerful for reconstructing the core oscillator, while also limiting direct extrapolation to the checkpoints and growth control of a differentiated somatic cell.

An immature oocyte is arrested in meiotic prophase. Hormonal stimulation can induce germinal-vesicle breakdown and progression towards a mature egg. Cytoplasm from a mature donor can induce maturation after injection into an immature recipient, establishing that a cytoplasmic activity can drive nuclear events. The activity was termed maturation-promoting factor, later also called M-phase-promoting factor. The foundational transfer experiments were performed in another frog species, Rana pipiens, so they should not be misattributed specifically to Xenopus.^[ref] Xenopus subsequently made the activity accessible to extensive extract assays and biochemical purification. Serial transfer also suggested amplification, rather than transfer of a fixed stoichiometric amount of a structural nuclear component.

Crushing unfertilized Xenopus eggs yields cytoplasm in which added nuclei can undergo characteristic mitotic events. Assay-guided fractionation of this material purified an activity capable of promoting early mitotic changes, making a connection between a cytoplasmic functional assay and specific protein components.^[ref] MPF is the complex of CDK1, historically Cdc2, with a mitotic cyclin such as cyclin B. The cyclin is a regulatory partner; CDK1 is the catalytic kinase. Nuclear-envelope breakdown, chromosome condensation and histone H1 kinase assays give complementary outputs. Co-purification with kinase activity is stronger when supported by immunodepletion and restoration of the active complex, because crude H1 phosphorylation is not uniquely specific to CDK1.

Cyclin synthesis and destruction provide the changing input. Cycling extracts show cyclin accumulation followed by destruction around mitotic exit. Blocking protein synthesis prevents repeated cycles; supplying appropriate cyclin material can restore mitotic entry, while a nondegradable cyclin prevents normal exit. These interventions distinguish association from a causal requirement for periodic synthesis and destruction.^[ref] Cyclin destruction is mediated by the anaphase-promoting complex, an ubiquitin ligase, followed by proteasomal degradation. It is therefore regulated proteolysis, not dilution or loss of kinase protein, that helps reset the mitotic machinery. Destruction of additional substrates coordinates chromosome separation with exit.

The response is not determined by cyclin concentration alone. Inhibitory CDK1 phosphorylation by Wee1 opposes activation; Cdc25 removes inhibitory phosphates. Active CDK1 promotes Cdc25 and suppresses Wee1, creating positive feedback. Xenopus extract experiments approaching the same sustained cyclin dose from an initially low or high kinase state found different maintained outputs, providing evidence of cell-cycle hysteresis experiment.^[ref] Monitoring endogenous cyclin destruction and the persistence of the added nondegradable input controls the possibility that the two histories merely leave different amounts of cyclin. This explains how gradual accumulation can produce a decisive transition rather than requiring every molecule to respond linearly.

Unfertilized eggs also reveal regulated arrest. They remain in meiotic metaphase II through cytostatic factor activity, which restrains the machinery for cyclin destruction. A fertilization-associated calcium signal releases that arrest, permitting cyclin destruction and meiotic completion. Experimentally adding calcium to an appropriately arrested extract can mimic this switch; a sham addition and monitoring cyclin and nuclear responses separate a genuine cycle transition from nonspecific extract damage.

Egg extracts additionally allow replication licensing to be separated from subsequent initiation. Added sperm chromatin can assemble nuclei and replicate, while transfer between extracts and manipulation of kinase state test why DNA is copied once per cycle. Licensing is permitted in a low-CDK state, while initiation-associated high CDK activity helps prevent relicensing before mitotic exit. A completed round of DNA synthesis must be distinguished from repair synthesis or simple nucleotide incorporation by appropriate product and template controls. The central contribution of Xenopus is experimental causality: a conserved cyclin–kinase system, feedback-controlled transitions, regulated proteolysis and replication control can be dissected in a cell-free cytoplasm rather than inferred only from correlations in dividing cells.

Retrovirus studies showed that a transmissible agent can cause a tumour and, more unexpectedly, that a viral transforming gene can derive from an ordinary cellular gene. This connected infectious cancer models to the molecular control of normal cell growth.

Peyton Rous transmitted a chicken sarcoma using material from a tumour and showed that a cell-free filtrate could transmit the disease. The agent could therefore be separated from intact tumour cells, arguing against simple transplantation of the malignant cells as the explanation.^[ref] This established a filterable tumour-causing agent, not yet its RNA genome, replication mechanism or a claim that every cancer is infectious. In repeating such an experiment, an uninoculated or sham-treated group and a preparation verified to lack viable donor cells would be essential comparators; host susceptibility and the preparation's retained infectivity would also matter.

The later Rous sarcoma virus system made transformation experimentally tractable. Infected cultured fibroblasts can lose contact inhibition, change morphology and form foci against a background of untransformed cells. Such assays distinguish efficient transformation from the slower and more complex endpoint of tumour formation in an animal. Temperature-sensitive transforming mutants allow the phenotype to be switched by changing conditions, while transformation-defective viruses can still replicate. This separates a transforming function from the basic machinery required to produce virus.

The retroviral life cycle explains persistent genetic change. Viral RNA is copied into DNA by reverse transcriptase; the resulting DNA can integrate into the cellular genome as a provirus. Demonstration of RNA-dependent DNA synthesis in Rous-virus particles gave biochemical support to this route of information transfer.^[ref] Template dependence, nuclease sensitivity and controls for contaminating cellular enzymes are needed to infer an RNA-templated polymerase rather than merely observe incorporation of labelled nucleotides. Integration then allows stable expression of viral genes through ordinary cellular transcription.

Comparison of transforming and transformation-defective viral genomes identified v-src as a transforming determinant. A crucial next question was whether src-like sequence existed only in tumour viruses. A probe enriched for transformation-specific viral sequences hybridized to DNA from normal birds, including multiple avian species. This showed that related information was present in normal cellular genomes, with evolutionary conservation rather than only virus-associated occurrence.^[ref] The cellular counterpart, c-src, is a proto-oncogene: a normally regulated cellular gene that can become oncogenic when its expression or activity is altered. Viral capture and modification of cellular growth-control information therefore explain why a virus can carry a potent growth-deregulating gene.

The Src product linked genotype to a biochemical mechanism. Its kinase activity was shown to phosphorylate tyrosine residues, establishing an unusual regulatory chemistry at the time and connecting transformation to deregulated protein phosphorylation.^[ref] The viral product has altered regulation compared with normal cellular Src, so persistent signalling can disrupt growth, adhesion and cytoskeletal control. The lesson is not that a normally expressed proto-oncogene is inherently malignant: context, dosage and regulatory state matter. Immunoprecipitate kinase assays need controls because an associated kinase could otherwise be mistaken for the precipitated protein's catalytic activity; biochemical and genetic tests strengthen the attribution.

Other retroviruses supplied a second route. Slowly transforming avian leukosis viruses need not carry a captured oncogene; integration near a cellular gene can alter its expression through viral regulatory sequences. Promoter-insertion studies of the MYC proto-oncogene demonstrated this principle.^[ref] Recurrent integrations near the same gene across independent tumours provide more persuasive evidence than one integration in one tumour, and expression and functional tests help distinguish a driver event from an incidental integration.

Together these experiments established separable mechanisms of viral oncogene acquisition and insertional oncogenesis, revealed cellular proto-oncogenes and identified deregulated signalling as a cause of transformation. They do not imply that retroviral integration explains every tumour, or that a transformed culture has all the properties of an invasive cancer. Their wider importance is that normal growth-regulatory genes, when altered, can drive malignant behaviour even outside the original infectious setting.