Cell Communication

Chapter 11:

Why do cells communicate?

Regulation - cells need to control cellular processes.

Environmental Stimuli - cells need to be able to respond to signals from their environment.

Cell Communication

Stages of C.S.

1. Reception - receiving the signal.

2. Transduction - passing on the signal.

3. Response - cellular changes because of the signal.

Reception

Transduction

Response

Reception

The target cell’s detection of a signal coming from outside the cell.

May occur by: Direct Contact Through signal molecules

Direct Contact

When molecules can flow directly from cell to cell without crossing membranes.

Plants - plasmodesmata Animals - gap junctions

Direct Contact

May also occur by cell surface molecules that project from the surface and “touch” another cell.

Signal Molecules

The actual chemical signal that travels from cell to cell.

Often water soluble. Usually too large to travel through

membranes.

Signal Molecules

Behave as “ligands”: a smaller molecule that binds to a larger one.

Receptor Molecules

Usually made of protein. Change shape when bind to a signal

molecule. Transmits information from the exterior

to the interior of a cell.

Receptor Molecules

1. G-Protein linked

2. Tyrosine-Kinase

3. Ion channels

4. Intracellular

G-protein linked

Plasma membrane receptor. Works with “G-protein”, an intracellular

protein with GDP or GTP.

G-protein

GDP and GTP acts as a switch. If GDP - inactive If GTP - active

G-protein

When active (GTP), the protein binds to another protein (enzyme) and alters its activation.

Active state is only temporary.

G-protein linked receptors

Very widespread and diverse in functions.

Ex - vision, smell, blood vessel development.

G-protein linked receptors

Many diseases work by affecting g-protein linked receptors.

Ex - whooping cough, botulism, cholera, some cancers

G-protein linked receptors

Up to 60% of all medicines exert their effects through G-protein linked receptors.

Tyrosine-Kinase Receptors

Extends through the cell membrane. Intracellular part functions as a

“kinase”, which transfers Pi from ATP to tyrosine on a substrate protein.

Mechanism

1. Ligand binding - causes two receptor molecules to aggregate.Ex - growth hormone

2. Activation of Tyrosine-kinase parts in cytoplasm.

3. Phosphorylation of tyrosines by ATP.

Intracellular Proteins

Become activated, cause the cellular response.

Tyrosine-Kinase Receptors

Often activate several different pathways at once, helping regulate complicated functions such as cell division.

Ion-channel Receptors

Protein pores in the membrane that open or close in response to chemical signals.

Allow or block the flow of ions such as Na+ or Ca2+.

Ion-channel Receptors

Activated by a ligand on the extracellular side.

Causes a change in ion concentration inside the cell.

Ex - nervous system signals.

Intracellular Signals

Proteins located in the cytoplasm or nucleus that receive a signal that CAN pass through the cell membrane.

Ex - steroids (hormones),NO - nitric oxide

Intracellular Signals

Activated protein turns on genes in nucleus.

Comment

Most signals never enter a cell. The signal is received at the membrane and passed on.

Exception - intracellular receptors

Signal-Transduction Pathways

The further amplification and movement of a signal in the cytoplasm.

Often has multiple steps using relay proteins such as Protein Kinases.

Protein Phosphorylation

The addition of Pi to a protein, which activates the protein.

Usually adds Pi to Serine or Threonine.

Protein Kinase

General name for any enzyme that transfers Pi from ATP to a protein.

About 1% of our genes are for Protein Kinases.

Amplification

Protein Kinases often work in a cascade with each being able to activate several molecules.

Result - from one signal, many molecules can be activated.

Secondary Messengers

Small water soluble non-protein molecules or ions that pass on a signal.

Spread rapidly by diffusion. Activates relay proteins.

Secondary Messengers

Examples - cAMP, Ca2+, inositol trisphosphate

cAMP

A form of AMP made directly from ATP by Adenylyl cyclase.

Short lived - converted back to AMP. Activates a number of Protein Kinases.

Calcium Ions

More widely used than cAMP. Used as a secondary messenger in

both G-protein pathways and tyrosine-kinase receptor pathways.

Calcium Ions

Works because of differences in concentration between extracellular and intracellular environments. (10,000X)

Used in plants, muscles and other places.

Inositol Trisphosphate (IP3)

Secondary messenger attached to phospholipids of cell membrane.

Sent to Ca channel on the ER. Allows flood of Ca2+ into the cytoplasm from

the ER.

Start here Or Start here

Cellular Responses

Cytoplasmic Regulation Transcription Regulation in the nucleus

(DNA --> RNA).

Cytoplasmic Regulation

Rearrangement of the cytoskeleton. Opening or closing of an ion channel. Alteration of cell metabolism.

Transcription Regulation

Activating protein synthesis for new enzymes.

Transcription control factors are often activated by a Protein Kinase.

Question

If liver and heart cells both are exposed to ligands, why does one respond and the other not?

Different cells have different collections of receptors.

Alternate explanation

Comment

Chapter focused only on activating signals. There are also inactivation mechanisms to stop signals.

Summary

Don’t get bogged down in details in this chapter.

Know - 3 stages of cell signaling. Know - At least one example of a

receptor and how it works (in detail).