Cell Communication

Overview• Essential for cells to have a way to

communicate with one another.

• Mechanisms are highly conserved = high levels of relatedness and connectedness.

• Evolved very early in the history of life.

S. cerevisiae• Yeast

• Can identify mates based on chemical signals.

• Occur in two forms: a and α.

• “a” factor binds to receptor proteins on α cells and vice versa.

• Binding causes cells to grow towards one another.

• Creates a hybrid offspring.

Signal Transduction Pathways• Series of steps in which surface

signals cause a change within a cell.

• Pathways are similar among various species:

– For example: yeast and mammals have very similar pathways, despite the fact that the common ancestor lived more than 1 billion years ago.

Communication • Local signaling: cells will secrete a local

regulator which will influence nearby cells.– For example: paracrine hormones and

neurotransmitters

• Long-distance signaling: uses hormones as chemical messengers that travel through the body via the bloodstream.– For example: endocrine hormones

Hormones• Chemical messengers.

• Wide variety of functions throughout the body.

• Vary in size and molecular type:– Hydrocarbons = ethylene

– Proteins = insulin

– Lipids = estrogen and testosterone

Cellular Junctions

• Cells can communicate via direct contact.

• Allows signaling molecules to travel from one cell to another through the cytosol.

What Happens When a Cell Receives A Signal??

• 3 main steps:

–Receptor recognition

–Signal transduction

–Response carried out by cell

1. Signal Reception

• Ligand binding = a small molecule binding to a larger one.

• Binding will often cause a conformational change to the ligand.

Types of Receptors

• Most are plasma membrane proteins.

• There are three major types that we’ll discuss:

1. G-protein-linked receptors

2. Tyrosine-kinase receptors

3. Ion channel receptors

G-Protein-Linked Recpetors

• Requires help of G-protein.• Many signaling molecules use this

receptor– Yeast mating factors– Epinephrine– Hormones– Neurotransmitters

• All G-proteins have a similar structure, although they vary in function.

G-Protein System Activation1. Signal molecules bind to extracellular

side of an inactive G-protein-linked receptor; causes a conformational change, and binding of another inactive G-protein.

2. Causes GTP to displace GDP; activates G-protein.

3. Active G-protein can bind to another protein (an enzyme for example).

4. Enzyme becomes activated and can catalyze reactions.

G-Protein Systems

• Widespread in cells throughout the body.– Embryonic development– Sensory reception (vision and smell)– Bacterial infections and botulism

• Many medications work by disrupting G-protein systems.

Tyrosine Kinase Receptors• Specialized for causing a cascading effect;

triggers several pathways at once.

• A portion of the receptor itself acts as an enzyme (tyrosine kinase).

• Tyrosine kinase catalyzes the phosphorylation of tyrosine (an amino acid) (phosphate comes from hydrolysis of ATP).

Tyrosine Kinase Activation1. Ligand binding causes dimerization of 2

receptor polypeptides.

2. Dimerization causes activation of tyrosine kinase on both polypeptides – each one will phosphorylate the tyrosine of the other.

3. This activates a receptor – recognized by relay proteins (intracellular) which attach to the phosphorylated tyrosines.

4. Causes a conformational change to the relay proteins which can trigger a cellular response.

Ion-Channel Receptors• Ligand-gated ion channels will open

or close in response to a chemical signal.

• This can selectively allow or block the entrance of certain ions into the cell.

• Very important functions in the nervous system.

Other Receptors• Not all receptors are membrane proteins.

• Some are dissolved in the cytosol.

• Can pass through the plasma membrane because they are hydrophobic.

• For example:

• Steroid hormones

• Thyroid hormones

• Nitrous oxide (NO) – important for vasodilation