Cell Communication & Signal Transduction

1. Cell Communication & Signal Transduction Ch. 11

2. Cell Communication • In multicelled organisms, individual cells must communicate and join with one another to create a harmonious organism. • Cell junctions can be classified in four functional groups: • Tight junctions • Desmosomes • Gap junctions • Plasmodesmata

3. Tight Junctions • Tight junctions are belts around epithelial cells that line organs and serve as a barrier to prevent leakage into or out of those organs (found in animal cells only).

4. Desmosomes • Desmosomes are found in many tissues and consist of clusters of cytoskeletal filaments from adjacent cells that are looped together. • They occur in tissues that are subjected to severe mechanical stress. • Found in animal cells only.

5. Gap Junctions • Gap junctions permit the passage of materials directly from the cytoplasm of one cell to the cytoplasm of an adjacent cell. • Found in animal cells only

6. Plasmodesmata • Plasmodesmata connect one plant cell to the next – they are analogous to gap junctions in animal cells.

7. Cell-to-Cell Recognition • Cell-to-cell recognition is the cell’s ability to distinguish one type of neighboring cell from another and is crucial to the functioning of a multicelled organism. • A feature of cells that aids in cell communication is the glycocalyx, which consists of oligosaccharides (small chains of sugar molecules) attached to integral proteins within the plasma membrane. • The glycocalyx is responsible for contact inhibition, the normal trait of cells to stop dividing when they become too crowded.

8. Cell-to-Cell Recognition

9. Signal Transduction • The signal transduction pathway relies on plasma membrane proteins in a multi-step process in which a small number of extracellular signal molecules produce a major cellular response. • Essentially, these pathways convert signals on a cell’s surface into cellular responses. • Three stages occur in this type of cell signaling: • Reception • Transduction • Response

10. Steps of Signal Transduction • In reception, the signal molecule, commonly a protein that does not enter the cell, binds to a specific receptor on the cell surface, causing the receptor molecule to undergo a change in conformation. • This conformational change leads to transduction – a change in signal form, where the receptor relays a message to a secondary messenger. • This secondary messenger, such as cyclic AMP (cAMP), induces a response within the cell.

11. Visual Overview of Cell Signaling

12. Receptors in the Plasma Membrane • There are three main types of membrane receptors • G-protein-linked • Tyrosine kinases • Ion channel

13. G-Protein Linked Receptor • A G-protein linked receptor is a membrane receptor that works with the help of a cytoplasmic G protein. • Ligand binding activates the receptor, which then activates a specific G-protein, which activates yet another protein in a signal transduction pathway. • Ligand is a term used to indicate a small molecule that specifically binds to a larger one – generally causing a conformation change in the shape of the larger molecule. • Epinephrine uses this sort of receptor.

14. G-Protein Linked Receptorhttp://bcs.whfreeman.com/thelifewire/content/chp15/15020.html

15. Tyrosine-Kinase Receptor • Tyrosine-kinase receptors react to the binding of signal molecules by forming dimers (a protein consisting of two polypeptides) and then adding phosphate groups to tyrosines on the cytoplasmic side of the receptor. • Relay proteins in the cell can then be activated by binding to different phosphorylated tyrosines, allowing this receptor to trigger several pathways at once. • Growth factors commonly use tyrosine-kinase receptors.

16. Tyrosine-Kinase Receptorhttp://www.wiley.com/college/fob/quiz/quiz21/21-15.html

17. Ion-Channel Receptorshttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__receptors_linked_to_a_channel_protein.html • Specific signal molecules cause ligand-gated ion channels in a membrane to open or close, regulating the flow of specific ions.

18. Apoptosis Apoptosis (programmed cell death) integrates multiple cell-signaling pathways. Cells that are infected or damaged have reached the end of their functional life and often enter a program of controlled cell suicide. Using signal transduction pathways, the cell components are disposed of in an orderly fashion, without damage to neighboring cells.

19. Signal Transduction Pathways • Pathways relay signals from receptors to cellular responses. • At each step in the pathway, the signal is transducted into a different form, commonly a conformational change in a protein. • Protein phosphorylation, a common mode of regulation in cells, is a major mechanism of signal transduction. • Many signal-transduction pathways include phosphorylation cascades, in which a series of protein kinases successively add phosphate groups to the next one in line, activating it. • Certain small molecules and ions are key components of signaling pathways (second messangers), such as cyclic AMP (cAMP) and Ca2+. • In response to a signal, a cell may regulate activities in the cytoplasm or transcription in the nucleus.