1. Cell Communication

2. 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.

3. 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.

5. 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.

6. 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

7. Hormones Chemical messengers.
Wide variety of functions throughout the body.
Vary in size and molecular type:
Hydrocarbons = ethylene
Proteins = insulin
Lipids = estrogen and testosterone

8. Cellular Junctions Cells can communicate via direct contact.
Allows signaling molecules to travel from one cell to another through the cytosol.

10. What Happens When a Cell Receives A Signal??
3 main steps:
Receptor recognition
Signal transduction
Response carried out by cell

11. 1. Signal Reception
Ligand binding = a small molecule binding to a larger one.
Binding will often cause a conformational change to the ligand.

12. Types of Receptors Most are plasma membrane proteins.
There are three major types that we’ll discuss:
G-protein-linked receptors
Tyrosine-kinase receptors
Ion channel receptors

13. 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.

15. G-Protein System Activation
Signal molecules bind to extracellular side of an inactive G-protein-linked receptor; causes a conformational change, and binding of another inactive G-protein.
Causes GTP to displace GDP; activates G-protein.
Active G-protein can bind to another protein (an enzyme for example).
Enzyme becomes activated and can catalyze reactions.

17. 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.

18. 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).

20. Tyrosine Kinase Activation
Ligand binding causes dimerization of 2 receptor polypeptides.
Dimerization causes activation of tyrosine kinase on both polypeptides – each one will phosphorylate the tyrosine of the other.
This activates a receptor – recognized by relay proteins (intracellular) which attach to the phosphorylated tyrosines.
Causes a conformational change to the relay proteins which can trigger a cellular response.

22. 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.

24. 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