Source: PCB3023L Assigned Reading, Protein Synthesis & Signaling
Difficulty: Intermediate | Prerequisites: Basic cell biology, protein synthesis (Weeks 1–3)
This material sits at the transition point between protein synthesis and protein function. Once a cell has made a protein, what does that protein do? In many cases it participates in signalling, the system by which cells detect environmental cues and translate them into actions such as growth, movement, or death. Signal transduction is one of the most heavily tested topics in cell biology because it ties together receptors, enzymes, gene expression, and disease (particularly cancer). You should already be comfortable with basic membrane structure, protein folding, and the central dogma before diving in.
Cells communicate by releasing signal molecules that bind to receptors on (or inside) target cells. This triggers cascades of intracellular events, often using phosphorylation or GTP-binding switches, that relay, amplify, and integrate signals to change cell behaviour. The same signal molecule can produce entirely different outcomes in different cell types, depending on which receptors and downstream proteins are expressed.
Cell signaling
The process by which cells communicate with one another through chemical or physical signals to coordinate behaviour. In simple terms, this is how cells "talk" to each other and make decisions together.
Signal transduction
The conversion of one type of signal into another, typically from an extracellular signal to an intracellular response. Think of it as a relay chain: a message arrives at the cell surface, gets translated into a different chemical language, and passes inward.
Ligand
A molecule (such as a hormone, growth factor, or neurotransmitter) that binds specifically to a receptor to initiate signalling. In simple terms, the "key" that fits into a receptor's "lock."
Receptor
A protein (on the cell surface or inside the cell) that detects a specific signal molecule and triggers a cellular response. Think of it as an antenna tuned to one frequency.
Effector protein
A protein at the end of a signalling pathway that carries out the cell's response, such as altering gene expression or metabolism.
Protein kinase
An enzyme that activates (or inactivates) a target protein by adding a phosphate group to it (phosphorylation).
Phosphatase
An enzyme that removes phosphate groups from proteins, reversing the action of kinases and switching signalling off.
GTP-binding protein (G protein)
A signalling protein that is active when bound to GTP and inactive when bound to GDP. Includes trimeric G proteins (coupled to GPCRs) and monomeric GTPases like Ras. Think of it as a molecular timer: GTP flips it on, GTP hydrolysis to GDP flips it off.
GPCR (G protein-coupled receptor)
A large family of cell-surface receptors that activate intracellular G proteins when they bind a ligand. These are the most common receptor type in human pharmacology.
EGFR (Epidermal Growth Factor Receptor)
A receptor tyrosine kinase that, when activated by EGF, dimerises and auto-phosphorylates to initiate downstream signalling (notably the MAP kinase and PI3K/AKT pathways).
Dimerisation
The pairing of two receptor molecules triggered by ligand binding, which activates their kinase domains.
MAP kinase pathway (MAPK)
A phosphorylation cascade downstream of receptors like EGFR, central to regulating cell growth, division, and differentiation.
PI3K/AKT pathway
A signalling pathway downstream of EGFR involved in promoting cell survival and growth. Phospho-AKT is a common readout of its activation.
Apoptosis
Programmed cell death. Cells that do not receive the right survival signals often undergo apoptosis. In simple terms, the cell's self-destruct programme, triggered when "stay alive" signals are missing.
Cells have several distinct ways of sending signals, and the differences matter because they affect speed, range, and specificity.
Endocrine signalling – Hormones travel through the bloodstream to reach distant target cells. This is slow (seconds to hours) but body-wide. Classic example: insulin released by the pancreas regulates glucose uptake in muscle and fat tissue far from the source.
Paracrine signalling – Signal molecules diffuse through the extracellular fluid to act on nearby cells only. Range is short (local tissue). Common in wound healing and inflammation, where cells at a damage site recruit their neighbours.
Autocrine signalling – A cell responds to signals it secretes itself. This creates a self-reinforcing loop and is frequently seen in cancer cells that drive their own survival and proliferation.
Neuronal signalling – Electrical impulses carry signals rapidly along axons, and neurotransmitters cross synapses to reach specific target cells. This is the fastest and most precisely targeted mode.
Contact-dependent signalling – Requires direct membrane-to-membrane contact between adjacent cells. Important during embryonic development, when neighbouring cells influence each other's fate through surface-bound signals.
The key variable across all modes is the trade-off between speed, distance, and specificity. Endocrine signalling reaches the whole body but is slow; neuronal signalling is fast but wired to specific targets.
Signal transduction starts when an extracellular signal molecule (the ligand) binds to a receptor protein on or in the target cell.
Cell-surface receptors bind large or hydrophilic ligands that cannot cross the plasma membrane on their own. Examples: growth factors, neurotransmitters, most peptide hormones.
Intracellular receptors bind small, hydrophobic ligands that can slip through the membrane. Examples: steroid hormones (oestrogen, testosterone, cortisol), thyroid hormone.
Once a receptor is activated, it initiates an intracellular signalling cascade that eventually changes the activity of effector proteins. Those effectors are the proteins that carry out the cell's response, whether that means switching genes on, altering metabolism, changing shape, or moving.
A critical point: the same signal can produce completely different outcomes in different cell types.
Acetylcholine slows heart rate in cardiac pacemaker cells.
Acetylcholine stimulates secretion in salivary gland cells.
Acetylcholine causes contraction in skeletal muscle.
The difference is not the signal molecule. The difference is which receptors and downstream components each cell type expresses. This is why "specificity" and "diversity" are tested together: the signal is the same, but the cellular context determines the response.
Cells rarely respond to a single signal in isolation. Survival, growth, and differentiation typically require the right combination of signals arriving at the same time. When those signals are absent, many cell types default to apoptosis. This is a safety mechanism: cells that are not receiving the correct instructions from their environment remove themselves rather than risk becoming harmful (this is directly relevant to understanding how cancer circumvents normal controls).
Once a receptor fires, the signal passes through a cascade of intracellular proteins. These pathways do more than simply pass the message along. They can:
Relay the signal deeper into the cell (from the membrane to the nucleus, for instance).
Amplify the signal, so that a small number of ligand molecules produces a large cellular response.
Integrate multiple signals arriving simultaneously to produce a coordinated outcome.
Distribute the signal to several different effectors at once, enabling complex, branching responses.
Regulate the response via feedback loops. Positive feedback sustains or strengthens the signal. Negative feedback dampens or oscillates it.
This combination of relay, amplification, integration, distribution, and feedback is what makes signalling pathways adaptable and precise.
Many intracellular signalling proteins work as binary switches, toggling between an active and an inactive state. Two major classes:
Phosphorylation-based switches
Protein kinases add phosphate groups to target proteins (phosphorylation), switching them on (or, in some cases, off).
Phosphatases remove phosphate groups, reversing the switch.
Kinase cascades, where one kinase phosphorylates and activates the next in a chain, are extremely common. They allow for both amplification (each kinase activates many copies of the next) and branching (a single kinase can phosphorylate several different targets).
GTP-binding protein switches
Active when bound to GTP, inactive when bound to GDP.
GTPase-activating proteins (GAPs) accelerate GTP hydrolysis to GDP, turning the switch off.
Guanine nucleotide exchange factors (GEFs) swap GDP for GTP, turning the switch on.
Trimeric G proteins work with GPCRs. Monomeric GTPases like Ras work downstream of receptor tyrosine kinases (including EGFR).
Both types of switch ensure that signals are transient: the default state is off, and active signals are self-limiting unless sustained by continued input.
EGFR (Epidermal Growth Factor Receptor) is the reading's central case study and ties together nearly every concept above.
Step-by-step activation:
The ligand EGF binds to the extracellular domain of EGFR.
Ligand binding causes EGFR to dimerise (two receptor molecules pair up).
Dimerisation activates the intrinsic tyrosine kinase domains on each receptor's cytoplasmic tail.
The kinase domains cross-phosphorylate each other's tyrosine residues (each receptor phosphorylates its partner).
The phosphorylated tyrosines act as docking sites for downstream signalling proteins.
Downstream cascades are initiated, including the MAP kinase pathway (via Ras, a GTP-binding protein) and the PI3K/AKT pathway.
Why this matters conceptually:
The initial activation of EGFR is a phosphorylation-based switch (tyrosine kinase activity).
Downstream, GTP-binding proteins like Ras are recruited, so both types of molecular switch are involved in the same pathway.
EGFR is directly relevant to cancer biology: mutations that cause EGFR to signal without ligand binding are oncogenic, and several cancer drugs (gefitinib, erlotinib, cetuximab) specifically target EGFR.
Key downstream readouts in the lab:
Anti-EGFR antibody detects total EGFR protein levels.
Phospho-AKT antibody indicates PI3K/AKT pathway activation.
Both are used in Western blotting and immunofluorescence (covered in the antibodies section).
Signal transduction is not abstract. Beta-blockers work by blocking adrenergic receptors (a form of signal transduction interference). Cancer therapies like Herceptin and EGFR inhibitors target specific receptors or kinases in signalling pathways. Insulin signalling failures underlie type 2 diabetes. Understanding these pathways is the foundation for modern pharmacology and molecular medicine.
Students often assume a signal molecule determines the response. It does not. The response depends on which receptors and intracellular machinery a given cell expresses. Acetylcholine in three different tissues produces three completely different outcomes.
Students often conflate "amplification" with "activation." Amplification means a small signal produces a large downstream effect (one kinase activating many copies of the next). Activation simply means switching a protein on. They are related but distinct.
Students sometimes think phosphorylation always activates a protein. It does not. Phosphorylation can also inactivate proteins, depending on the site and the protein.
Students sometimes treat EGFR as a GTP-binding protein switch because Ras is involved. The initial activation of EGFR is phosphorylation-based (tyrosine kinase). Ras is recruited downstream.
⚠️ Be ready to distinguish the five modes of cell communication (endocrine, paracrine, autocrine, neuronal, contact-dependent) and give an example of each.
⚠️ Know why the same ligand can produce different responses in different cell types.
⚠️ Be able to walk through the EGFR signalling cascade step by step, from ligand binding to downstream pathway activation.
⚠️ Understand the difference between phosphorylation-based and GTP-binding molecular switches, and know which class EGFR's initial activation belongs to.
⚠️ Be able to explain how positive and negative feedback regulate signalling pathways.
True or False: Endocrine signalling acts only on cells immediately adjacent to the signalling cell.
Fill in the blank: A protein kinase activates its target by adding a ______ group.
True or False: EGFR's initial activation mechanism is a GTP-binding switch.
Fill in the blank: In the absence of appropriate survival signals, many cells undergo ______.
True or False: Acetylcholine produces the same cellular response in every tissue.
(Answers: 1. False, that describes paracrine or contact-dependent signalling. 2. Phosphate. 3. False, it is phosphorylation-based. 4. Apoptosis. 5. False, the response depends on the cell type's receptors and downstream machinery.)
Q: Compare and contrast endocrine and paracrine signalling in terms of range, speed, and provide one example of each.
A: Endocrine signalling is long-range (hormones travel through the bloodstream to distant targets) but relatively slow. Example: insulin from the pancreas regulating glucose uptake throughout the body. Paracrine signalling is short-range (signal molecules diffuse locally through extracellular fluid) and faster at the local level. Example: growth factors released during wound healing that recruit nearby cells.
Q: Explain why acetylcholine produces different effects in cardiac pacemaker cells, salivary gland cells, and skeletal muscle cells.
A: Each cell type expresses different receptor subtypes and downstream signalling components. The signal molecule is the same, but the intracellular machinery that interprets it differs. This means the cellular context, not the ligand, determines the response.
Q: Walk through the EGFR activation cascade, starting from EGF binding.
A: EGF binds the extracellular domain of EGFR, causing receptor dimerisation. The paired kinase domains cross-phosphorylate tyrosine residues on each other's cytoplasmic tails. These phosphotyrosines serve as docking sites for downstream proteins, initiating the MAP kinase pathway (via Ras) and the PI3K/AKT pathway. The initial switch is phosphorylation-based; GTP-binding proteins like Ras are recruited downstream.
Q: What is the functional difference between a kinase and a phosphatase in a signalling pathway?
A: A kinase adds a phosphate group to a target protein (phosphorylation), typically activating it (though it can also inactivate). A phosphatase removes the phosphate group, reversing the modification. Together they act as an on/off switch for protein activity in signalling cascades.
Q: Why do signalling pathways use molecular switches rather than permanent activation?
A: Transient, reversible switches allow cells to respond quickly to changing conditions and shut signals off when they are no longer needed. Permanent activation would be uncontrollable and is, in fact, a hallmark of disease states such as cancer (e.g. constitutively active Ras).
This material connects directly to protein synthesis (Weeks 1–3): the proteins discussed here (receptors, kinases, G proteins) are the functional products of gene expression. Understanding how they are made (transcription, translation, folding) is prerequisite to understanding what they do.
Cell signalling also connects forward to cancer biology: most oncogenes encode mutant signalling proteins (Ras, EGFR, PI3K) that are stuck in the "on" state, and tumour suppressors often encode proteins that normally shut signalling off.
The antibody-based detection methods covered in the companion notes (Western blotting, immunofluorescence) are the experimental tools used to study these signalling events in the lab.
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