lecture 11, 12, 13- rafts and membrane potentials

What are the two key functions of membranes mentioned in the lecture?

Nerve impulses and signaling.

Lipid rafts can bring proteins together or keep them apart, and proteins can be easily moved in and out of them through covalent modifications like ___ or ___

GPI anchor attachment, prenylation

What experimental technique attaches a sensitive electrode to a patch of membrane to measure current from ion channels?

Patch clamping.

In a patch clamp recording, what does the top level with zero current represent?

All channels in the patch are closed.

In a patch clamp recording, the fact that excited (open) levels are at fixed positions indicates what about the channels?

All channels have the same current when open.

The opening and closing of individual ion channels is essentially _, but the average rates are specific to the channel type.

random

How do clustering agents like hormones trigger a chemical chain reaction using lipid rafts?

They cause individual rafts (and the proteins within them) to cluster together, bringing the proteins into close contact.

In ligand-mediated endocytosis, what protein binds to the membrane raft and inserts halfway to make it curved?

Caveolin.

After caveolin curves the membrane, what proteins does it recruit to make a coat around the forming caveolae?

More proteins like cavin and clathrin.

According to the fractional conductance graph, at what membrane potential do voltage-gated channels start to open?

At approximately -40 mV.

According to the fractional conductance graph, under what condition is the maximum ion flow achieved?

When the membrane potential is zero or positive.

What is the typical normal resting potential for a cell as indicated on the fractional conductance graph?

Approximately -60 mV.

What happens to the frequency and duration of ion channel openings as the membrane potential is increased from -10 mV to +50 mV?

The channels open more frequently and for longer durations.

Voltage-gated Na+ channels have a plug that closes the channel after it has been open for about how long?

Approximately 1 ms.

What is the first step in the activation of a voltage-gated Na+ channel?

Initial depolarization causes the movement of voltage-sensing alpha helices, opening the channel.

The period when the channel-inactivating segment is displaced and the gate closes, making the channel unable to open, is called the _.

refractory period

What is the main difference in kinetics between voltage-gated K+ channels and Na+ channels?

The opening and closing of the K+ channel is slower.

Because of its slower kinetics, the voltage-gated K+ channel is sometimes called a _ K+ channel.

delayed

How do cells primarily maintain a negative membrane potential?

Positively charged K+ ions leak out of the cell through channels, leaving a net negative charge inside.

In the context of nerve impulses, a stronger signal is obtained by _.

more rapid action potentials

At a synapse, what event is triggered by the arrival of an action potential?

It opens Ca2+ channels, triggering exocytosis of vesicles containing neurotransmitters.

What primarily determines the resting potential in almost all human cells?

The K+/Na+ ratio, specifically the flow of K+ ions out of the cell.

What type of channels are open all the time in a resting cell, allowing K+ to leak out?

Resting K+ channels (or K+ leak channels).

Approximately what percentage of our total ATP consumption is used to power the Na+/K+ pump?

About 25%.

What is the threshold potential that must be reached to trigger the opening of voltage-gated Na+ channels and start an action potential?

-40 mV.

The initial phase of an action potential involves a _ feedback loop where Na+ influx opens more Na+ channels.

positive

During an action potential, after Na+ channels inactivate, what channels open to repolarize the membrane?

Delayed K+ channels.

The outflow of K+ ions during repolarization is strong enough to briefly _ the membrane.

hyperpolarize

Why can an action potential only travel 'forwards' along an axon?

The preceding section of the membrane is in a refractory period and its Na+ channels cannot be opened.

Despite the large change in membrane potential, one action potential has very little effect on the overall _ of Na+ and K+ in the cell.

concentrations

How do myelin sheaths increase the speed of action potential travel?

They allow the potential to jump between gaps called nodes of Ranvier, where the voltage-gated channels are concentrated.

What neurodegenerative disease is caused by the loss of myelin in some areas?

Multiple sclerosis.

The poison tetrodotoxin, from puffer fish, works by blocking which specific channels?

Voltage-gated sodium channels.

How does the poison batrachotoxin from frog skin affect sodium channels?

It binds to them and keeps them permanently open.

The total potential (Delta-p) across a membrane is the sum of the electric potential gradient and the _ gradient.

chemical (pH)

Term: Electric potential gradient (Delta-psi)

The difference in electrical charge across the membrane, creating a voltage.

Term: Chemical gradient (Delta pH)

The difference in the actual concentration of ions (protons) across the membrane.

Formula: Proton motive force

Delta p = Delta psi - Delta pH

What is the main reason cancer can develop, in the context of cell signaling?

Cellular proliferation signals are not being turned off.

G-proteins are switched on when bound to ___ and switched off ___ when bound to .

GTP; GDP

What type of enzyme is used to turn on a G-protein signal by promoting the dissociation of GDP?

GEF (Guanine nucleotide exchange factor).

What type of enzyme is used to turn off a G-protein signal by hydrolyzing GTP to GDP?

GAP (GTPase-activating protein).

What is the first step in activating receptor-linked kinases (RTKs)?

A ligand binds to two neighboring monomers, causing them to form a dimer.

Once RTKs form a dimer, what process turns on the receptor's kinase activity?

The kinase domain of each receptor adds a phosphate group to its partner (autophosphorylation).

The newly added phosphates on an activated RTK act as docking sites for what?

Other signaling proteins inside the cell, such as modular adaptor proteins.

In RTK signaling, the adaptor protein Grb2 contains an SH2 domain that recognizes what?

Phosphotyrosines.

In RTK signaling, the adaptor protein Grb2 contains an SH3 domain that recognizes what?

Polyproline helices.

The GEF protein _ has a proline-rich arm that allows it to plug into Grb2's SH3 domain.

Sos

How does phosphorylation typically activate a kinase?

The negative phosphate makes a disordered activation loop more rigid, improving substrate binding.

G-protein coupled receptors (GPCRs) are structurally characterized by _ transmembrane helices.

seven

The GPCR acts as a _ for the G-alpha subunit.

GEF (Guanine nucleotide exchange factor)

In GPCR signaling, ligand binding causes a conformational change that opens a groove for G-alpha, allowing ___ to leave and ___ to bind.

GDP; GTP

Trace the basic GPCR signaling pathway starting from an activated G-protein.

G-protein activates adenylyl cyclase, which synthesizes cAMP, which activates protein kinase A (PKA).

What is the function of the G-alpha s (stimulatory) subunit?

It activates the cAMP-dependent pathway by turning on adenylyl cyclase.

What is the function of the G-alpha i (inhibitory) subunit?

It inhibits the cAMP-dependent pathway by inhibiting adenylyl cyclase.

What is the function of the G-alpha q subunit?

It stimulates phospholipase C-beta, which splits PIP2 into DAG and IP3.

How does the cholera toxin cause diarrhea?

It ADP-ribosylates G-alpha s proteins, locking them in the ON state, leading to continuous cAMP production and release of water and ions from intestinal cells.

How does the whooping cough toxin work?

It ADP-ribosylates a G-alpha i subunit, preventing the inhibition of adenylyl cyclase, leading to excessive cAMP production.

What protein is involved in turning off GPCR signaling via phosphorylation, leading to endocytosis and recycling?

Arrestin.

What is a major advantage of using ion channels for signaling?

They can turn on a signal very rapidly.

What is a major disadvantage of using ion channels for signaling?

Reversing the signal requires removing all the ions, which can use a lot of energy.

What signaling pathway is often used when speed is essential?

Ion channels, and sometimes GPCRs.

What signaling pathway is often used when specificity and regulation are most important?

Kinase-based pathways (like RTKs).

How do hydrophobic ligands like steroids enter the cell?

They can diffuse directly across the cell membrane.

In the cytoplasm, an unbound steroid receptor is typically inactive because it is bound to what kind of protein?

A large cytoplasmic protein, such as Hsp90.

After a hydrophobic ligand binds its cytoplasmic receptor, what does the activated receptor do?

It moves to the nucleus and binds to response elements on DNA, acting as a transcription factor.