MCDB 165A Cell Signaling Flashcards

For many eukaryotic cells, survival is…

not the default. Cells will undergo apoptosis unless they receive signals (e.x growth factors).

In addition to survival signals, there are signals that affect cell behavior, like cell division:

Signaling important for cell division during development. Also, need to suppress cell division in mature organisms, except where they are carefully regulated in stem cells.

Differentiation Signal

In development, cells start out undifferentiated. Need to receive signal to alters their gene expression and become specialized.

Endocrine Signaling

Cells can receive signals from very far away. Signal (hormone) could be made in one organ that travels through the blood and reaches a cell in a different part of the body. Often there is a multi-system response.

Paracrine Signaling

Signals move through a short distance in the environment. In development, this allows tissues to coordinate their differentiation. Allows signaling to nearby cells, not just nextdoor neighbors.

May be some active mechanisms for moving these signals.

Autocrine Signaling

When a cell signals to itself. Cancer cells secrete growth factors to maintain themselves/promote proliferation.

Juxtacrine Signaling

Occurs only with two cells that are side-by-side and close enough to physically touch.

Signals are on cell surface, so two cells must be in contact.

Signals can affect a cell at the same time and…

potentially interact.

Signal Transduction Basic Steps

When the signal interacts with a receptor on the cell surface, it changes conformation and its activity/strucuture is changed. This change in activity in the receptor activates a change in activity in another internal protein/molecule. This causes a cascade that results in a change in the cell.

Each protein in a pathway alters the conformation…

of the next protein (phosphorylation is an example).

Pathways are not always linear: pathways can…

converge, diverge, and cross-talk.

Converge

Two signals must be received to get a specific outcome. Receptors sensing different signals can activate different downstream proteins that may interact, tuning the cellular response.

**Online information shows slightly different definition: signals from a variety of unrelated receptors converge to activate a common effector.

Diverge

One signal may have one outcome (transcription) and a second outcome (cytoskeleton).

As an example of this, there may be one pathway activated by a signal that initiates a rapid response and then a slower signaling pathway activated by that same signal that shuts off the response.

Cross-Talk

You may have two pathways that seem to be acting separately, but can affect each other.

In other words, each signaling pathway can work independently, but if they are working at the same time there may be some interaction where one of the pathways gets altered/shut off.

The cytoplasmic environment is very crowded, creating signaling challenges. But, cells have evolved mechanisms to…

make transduction sensitive when it needs to be, efficient, and tailored to the context of the cell. One of these mechanisms is scaffold proteins.

Scaffold Proteins

Interacts with components of the pathway and concentrates them in one area so they are all near each other. Often, the cell needs the scaffold protein to create efficient transduction.

All in all, these simplistic renderings of transduction pathways occur in a…

complicated environment.

Signals that go through the membrane must be…

small and hydrophobic.

Nuclear Hormones (Steroid Hormones)

Often secreted by glands into the bloodstream and affect cells throughout the body.

Steroid Hormone Signaling Pathway

Steroid hormones diffuse through the membrane and the receptors are inside the cytoplasm. If the cell expresses a receptor, the steroid physically interacts and moves the complex from the cytoplasm through a nuclear pore where it can interact with DNA.

G-protein Receptors

These receptors are on the cell membrane and are also called 7 transmembrane receptors. They are the largest family of cell surface receptors.

The main set of proteins that function downstream to transduce the GPCR response are called…

heterotrimeric G-proteins.

Heterotrimeric G-Proteins

Complex of three proteins (alpha, beta, and gamma) that interact with each other and the receptor.

Alpha and gamma are…

lipid-anchored proteins, so they are inserted into the membrane.

The beta protein interacts with alpha and gamma, so it is close to these molecules and thus the membrane/receptor.

1) Inactive State

Both the receptor and heterotrimeric G protein complex are inactive and not bound to each other; the α subunit is GDP-bound.

When the g-alpha protein is complexed with GDP, it is…

inactive and interacts with the beta and gamma, not the receptor.

2) Activation of the receptor by a ligand enables the…

heterotrimeric G protein complex to associate with the receptor.

When the receptor binds a ligand, the inside of the GPCR can now interact with the heterotrimeric complex (specifically the alpha protein).

3) Binding to the activated receptor enables the…

alpha subunit to exchange its GDP for GTP; the α subunit dissociates from the receptor and the β/γ subunits.

The alpha subunit and beta/gamma subunits can interact with many downstream effectors, including:

an enzyme that makes many small molecules to amplify the signal, a channel that allows ions to pass into the cell, or a lipase that cleaves phospholipids.

The alpha subunit has an internal clock called a…

GTPase. Alpha is active when bound to GTP, but its GTPase activity hydrolyzes the GTP into GDP, making the alpha protein inactive.

The inactivated alpha protein will then be reassociated to the…

beta and gamma subunits.

Downstream affects of g-alpha subunit (enzymes):

Activated g-alpha can interact/activate an enzyme (beta/gamma may be nearby interacting with something else). This interaction can catalyze a reaction that amplifies the signal by making secondary messangers.

For one receptor activated, one g-alpha was activated, and thus one enzyme activated. Now, there are many downstream molecules (cAMP) that can go on and transduce the signal.

Second Messengers

Small molecules generated by a signal transduction pathway that amplify the signal. So, the cell becomes more sensitive to a signal.

If a signal activated one receptor, which caused downstream activation of proteins in a 1:1 ratio, then you could only amplify the pathway by…

adding more signals. The strength of signaling would be proportional to the amount of signal.

Common Second Messengers:

Calcium (channels open, allowing Ca to enter and affect proteins), cAMP/cGMP (many proteins that depend on cAMP and cGMP), and diacylglycerol and IP3 (generated by a lipase, which cleaves a glycerophospholipid).

cAMP Activation Example

g alpha protein interacts with adenylate cyclase, which makes cAMP from ATP.

A PKA kinase is only active when bound to cAMP. So, activation of one receptor can generate many cAMP molecules which in turn activate several molecules of PKA which can phosphorylate complexes in the nucleus (for as long as the G protein is active).

Kinase

Enzyme that phosphorylates (adds a phosphate group) to a protein.

Most common ones are serine/threonine kinases (phosphate group added to amino acid serine/threonine) and tyrosine kinases (phosphate group added to amino acid tyrosine).

The phosphorylated and unphosphorylated versions of a protein…

have different activity. Phosphorylation acts as a switch.

Receptor Tyrosine Kinase Activation

Many different RTKs tend to be receptors for growth factors, promoting cell survival/proliferation. These receptors can be kinases, but when there is no signal their kinase activity is off.

When the signal binds, two receptors dimerize (homodimer or heterodimer), and their kinase activity is active. One receptor subunit phosphorylates the other, called cross-phosphorylation (the main target of the kinase is a receptor tyrosine kinase).

Once the intracellular portions of the receptor are phosphorylated, their structures have changed and they can interact with downstream effectors.

So, the key steps in the receptor tyrosine kinase activation are...

dimerization and cross-phoshporylation.

Herceptin

Blocks dimerization of the EGF Tyrosine Kinase Receptor. An HER2 is an RTK receptor whose overactivation is associated with a deadly form of breast cancer.

A small antibody drug (herceptin) can interact with HER2 and block its interaction with its dimerization partner HER3, preventing activation of the pathway.

Activation of an RTK can lead to activation of a downstream signaling molecule called…

RAS. When an RTK is cross-phosphorylated, it can interact with the adaptor protein Grb2 (which binds exclusively to the phosphorylated RTK). Grb2 then recruits another protein called SOS, which is an activator of RAS.

RAS binds to GDP and GTP (like G-alpha) and is also a lipid-modified protein that is inserted into the membrane.

When RAS is GDP bound it is inactive and when it is GTP bound it is active. SOS interacts with RAS to cause it to exchange GDP for GTP, thereby activating it.

RAS Mutations

RAS has to hydrolyze GTP to enter its inactive state, and there are mutations in RAS that compromise this ability, making it always active and continuously transducing the cell survival/proliferation signal. This mutation is a RAS oncogene.

RAS Downstream Effects

GTP-bound RAS can activate RAF (MAP kinase kinase kinase), which then phosphorylates a MAP kinase kinase, which then phosphorylates a MAP kinase.

This MAP kinase can go into the nucleus and affect transcription or things in the cytoplasm.

Why have this kinase cascade and not just one kinase?

More steps in a signal transduction pathway, the more opportunities for regulation, cross-talk, and convergence.

MAP Kinase Pathway is a common motif (not exclusive to RTKs)…

For mating in yeast, there is a Map kinase cascade that responds to activation of a GPCR.

Map kinase cascades can be nodes for potential cross-talk/convergence. For instance, a Map kinase cascade can respond to both a GPCR and an RTK.