Cell Communication & Signal Transduction Ch. 11

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

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). tight junctions – animals – membranes of neighboring cells are actually fused; prevent leakage of extracellular fluid across a layer of epithelial cells

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. desmosomes – animals; “anchoring junctions” – function like rivets, fastening cells together in strong sheets (are reinforced by intermediate filaments made of keratin)

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 gap junctions – animals; “communicating junctions” provide cytoplasmic channels between adjacent animal cells – see page 134

Plasmodesmata Plasmodesmata connect one plant cell to the next – they are analogous to gap junctions in animal cells. plasmodesmata – plants; channels that allow cytosol to pass through and connect the living contents of adjacent cells (see page 134)

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.

Cell-to-Cell Recognition

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

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.

Visual Overview of Cell Signaling 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.

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

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.

G-Protein Linked Receptor http://bcs. whfreeman In the absence of the extracellular signal molecule specific for the receptor, all three proteins are in inactive form. When the signal molecule binds to the receptor, the receptor changes shape in such a way that it binds and activates the G protein. A molecule of GTP replaces the GDP on the G protein. The active G protein moving freely along the membrane binds to and activates the enzyme which triggers the next step in the pathway leading to the cell’s responses. The G protein then catalyzes the hydrolysis of it GTP and dissociates from the enzyme, becoming available for future use.

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. A dimer is a protein consisting of two polypeptides.

Tyrosine-Kinase Receptor http://www. wiley When signal molecules (such as a growth factor) attach to their binding sites, two polypeptides aggregate, forming a dimer. Using phosphate groups from ATP, the tyrosine-kinase region of each polypeptide phosphorylates the tyrosines on the other polypeptide. Now fully activated, the receptor protein can bind specific intracellular proteins. These relay proteins attach to particular phosphorylated tyrosines and in the process are themselves activated. Each of these proteins can then initiate a signal transduction pathway leading to a specific cellular response. Tyrosine-kinase receptors often activate several different signal-transduction pathways at once.

Ion-Channel Receptors http://highered. mcgraw-hill Specific signal molecules cause ligand-gated ion channels in a membrane to open or close, regulating the flow of specific ions. This receptor is a transmembrane protein in the plasma membrane that opens to allow the flow of a specific kind of ion across the membrane when a specific signal molecule binds to the extracellular side of the protein.

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. 18

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.