BISC 1303: Chapter 9 - Cellular Communication

Ligand

a molecule produced by a signaling cell that binds with a specific receptor, delivering a signal in the process

releases from “signaling cells”

bind to “receptor proteins” on “target cells”

4 types of Signaling Mechanisms

1. Autocrine

2. Direct Signaling

3. Paracrine

4. Endocrine

Autocrine

ligand binds to receptor on signaling cell

Direct Signaling

ligand travels directly from cytoplasm to cytoplasm from signaling to target cell

Paracrine

ligand binds to receptors on NEARBY cells

rapidly degraded ligand ensures signal is local

Endocrine

ligand binds to receptors on DISTANT cells

stable ligand ensures it can travel across body

2 types of Receptors

1. Intracellular Receptor

2. Cell surface receptor

Intracellular Receptor

receptor protein in cytoplasm of cell

ligands for these receptors must be small and hydrophobic to diffuse across membrane

E.G. steroid hormones

Cell surface receptors

receptor protein on surface of cell

anchored in plasma membrane

have 3 distinct domains:

1. extracellular - exposed to outside of cell, binds ligand

2. transmembrane - spans membrane

3. cytoplasmic - exposed to cytoplasm

ligands for these receptors are water-soluble

E.G. small molecules or proteins

Signal Transduction

binding of ligand leads to change in shape of receptor protein

• behaves different

E.G. an intracellular receptor may enter nucleus and cause genes to be turned on/off

E.G. may lead to release/creation of a second messengers

E.G. a cell surface receptor may phosphorylate proteins

Second messenger

a small, non-protein molecule that propagates a signal within the cell after activation of a receptor causes its release

Phosphorylation

modification of protein by addition of phosphate group(s) from ATP

catalyzed by a class of enzymes called “kinases”

Dephosphorylation

removal of phosphate group(s) from a protein

catalyzed by a class of enzymes called “phosphatases”

Based on its phosphorylation state…

the function of a protein can be switched on or off

Signal Transduction can ultimately result in

altered cell behavior

cell growth/division

apoptosis

Apoptosis

programmed cell death

Singled-celled yeasts…

secrete a “mating factor” ligand to find other yeast cells to mate with

Some bacteria can communicate…

E.G. Quorum sensing

• some bacteria can form structures called biofilms but this requires high cell density to form, would waste energy to try to form with few bacteria

• released ligands (called autoinducers) communicate cell density to other bacteria

low ligand density - ligand diffuses

high ligand density - signaling pathway activated, formation of biofilm

Biofilm

a matrix of proteins and sugars that protects colonies of pathogenic bacteria from the host immune system