Cell Communication AP Biology Mrs. Ramon

Intro. to Cell Communication Essential for multicellular organisms. Cells must communicate to coordinate their activities. Also important for many unicellular organisms. Some universal mechanisms Variety of signals: chemical, electromagnetic and mechanical

Evolution of Cell Signaling Cell communication is crucial in reproduction The yeast Saccharomyces cerevisiae identifies its mates by chemical signaling Two sexes: a and alpha () Each type secretes a specific signaling molecule, a factor and alpha factor

Binding of mating factors to their respective receptors causes the cells to shmoo Two opposite cells fuse, or mate The a/ cell contains the genes of both cells.

Evolution of Cell Signaling Signal-transduction pathways Process by which a signal on a cell’s surface is converted into a specific cellular response in a series of steps. Signaling molecules evolved first in ancient prokaryotes and were then adopted for new uses by single-celled eukaryotes and multicellular descendents.

Cell signaling has remained important in the microbial world. Myxobacteria, soil-dwelling bacteria, use chemical signals to communicate nutrient availability. When food is scarce, cells secrete a signal to other cells leading them to aggregate and form thick-walled spores.

Methods of Cell Communication Multicellular organisms also release signaling molecules that target other cells. Some transmitting cells release local regulators that influence cells in the local vicinity. Paracrine signaling occurs when numerous cells can simultaneously receive and respond to growth factors produced by a single cell in their vicinity. Fig. 11.3a1

Methods of Cell Communication In synaptic signaling, a nerve cell produces a neurotransmitter that diffuses to a single cell that is almost touching the sender. An electrical signal passing along the nerve cell triggers secretion of the neurotransmitter into the synapse. Nerve signals can travel along a series of nerve cells without unwanted responses from other cells.

Methods of Cell Communication Plants and animals use hormones to signal at greater distances. In animals… Specialized endocrine cells release hormones into the circulatory system In plants… Vessels or by diffusion in air

Methods of Cell Communication Direct contact Signaling substances dissolved in the cytosol pass freely between adjacent cells Cells may also communicate via direct contact between substances on their surfaces.

Stages of Cell Communication Three stages: Reception: a chemical signal binds to a cellular protein, typically at the cell’s surface. Transduction: binding leads to a change in the receptor that triggers a series of changes along a signal-transduction pathway. Response: the transduced signal triggers a specific cellular activity.

Reception and Transduction Binding of a signaling molecule (ligand) to its specific receptor induces a conformational change Most ligands are water-soluble and large You got a problem with that?!? Influence cell activities by binding to receptor proteins on the plasma membrane. Three major types of receptors: G-protein-linked receptors Tyrosine-kinase receptors Ion-channel receptors.

Types of Receptors A G-protein-linked receptor consists of a receptor protein associated with a G-protein on the cytoplasmic side. The receptor consists of seven alpha helices spanning the membrane. Include:yeast mating factors, epinephrine, other hormones, and neurotransmitters.

The G protein acts as an on-off switch. If GDP is bound, the G protein is inactive. If GTP is bound, the G protein is active.

G-proteins in Action

Inactivating G-proteins The G protein can also act as a GTPase enzyme and hydrolyzes the GTP, which activated it, to GDP. This change turns the G protein off. The whole system can be shut down quickly when the extracellular signal molecule is no longer present.

Tyrosine-kinase Receptors An individual tyrosine-kinase receptor consists of several parts: an extracellular signal-binding sites, a single alpha helix spanning the membrane, and an intracellular tail with several tyrosines.

Activation of Ty-K Receptors When ligands bind to two receptor polypeptides a dimer is formed. These add phosphates to the tyrosine tails of the other polypeptide Activate a variety of relay proteins

Ion Channels Protein pores that open or close in response to a chemical signal. This allows or blocks ion flow Binding by a ligand changes the protein’s shape and opens the channel Ion flow changes the concentration inside the cell When the ligand dissociates, the channel closes.

Transcription Factors Testosterone, like other hormones, travels through the blood and enters cells throughout the body. In the cytosol, they bind and activate receptor proteins. These activated proteins enter the nucleus and turn on genes that control male sex characteristics.

Signal Transduction Pathways

Secondary Messengers A common secondary messenger is cAMP Binding of the receptor activates a G protein that activates adenylyl cyclase in the plasma membrane. The cAMP diffuses and activates a serine/threonine kinase, which phosphorylates other proteins.

Cellular Response Stimulation of glycogen breakdown by epinephrine involves a G-protein-linked receptor, a G Protein adenylyl cyclase and cAMP, and several protein kinases before glycogen phosphorylase is activated.

Other signaling pathways do not regulate the activity of enzymes but the synthesis of enzymes or other proteins. Activated receptors may act as transcription factors that turn specific genes on or off in the nucleus.

The response of a particular cell to a signal depends on its particular collection of receptor proteins, relay proteins, and proteins needed to carry out the response. Fig. 11.18 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Two cells that respond differently to the same signal differ in one or more of the proteins that handle and respond to the signal. A single signal may follow a single pathway in one cell but trigger a branched pathway in another. Two pathways may converge to modulate a single response. Branching of pathways and interactions between pathways are important for regulating and coordinating a cell’s response to incoming information.

Rather than relying on diffusion of large relay molecules like proteins, many signal pathways are linked together physically by scaffolding proteins. Scaffolding proteins may themselves be relay proteins to which several other relay proteins attach. Enhances speed and accuracy of signal transfer between cells.