Cell Communication

Chapter 11

An overview of Cell Signaling

Signal-transduction pathway

• Process by which a signal on a cell’s surface is converted into a specific cellular response is a series of steps called signal-transduction pathway

Evolutionary connection

• Scientists think that signaling mechanisms evolved in ancient prokaryotes and single celled eukaryotes• They were adopted for new uses

by multicellular descendents

• Local regulator – a substance that influences cells in its vicinity• Transmitting cells can secrete

molecules of a local regulator to communicate with nearby cells

Paracrine signaling

• Paracrine signaling is a type of local signaling in animals in which numerous cells can simultaneously receive and respond to the signal of a single cell in their vicinity

Examples of local regulators that uses paracrine signaling

• Growth factors in animal cells are compounds that stimulate nearby target cells to grow and multiply

Synaptic Signaling

• Specialized type of local signaling that occurs in animal nervous systems• An electrical signal that is

transmitted the length of a nerve cell to its target cell without touching the target cell

Example of synaptic signaling• Nerve cells produce a

chemical signal called a neurotransmitter that diffuses to a single target cell that is almost touching the signaling cell

Long distance signaling

• Plants and animals use chemicals called hormones for signaling

• In animals – hormonal signaling is called endocrine signaling–Hormone molecules are

released into vessels of the circulatory system and travel to other parts of the body

• In plants – sometimes they move through

vessels–More often they are diffused

through the air as a gas•Example: plant hormone ethylene is a gas that helps promote fruit ripening

Direct Contact• Both plants and animals have cell

junctions that promote the direct contact between 2 cells• Signaling substances dissolve in the

cytosol and can pass freely between adjacent cells• Animal cells can communicate directly

between the molecules on their cell surfaces

Three stages of cell signaling

• Earl W. Sutherland discovered how the hormone epinephrine acts on cells

• Sutherland suggested that cells receiving signals went through three processes:–Reception–Transduction–Response

Reception

• First step when the target cell detects a signal coming from outside the cell• Chemical signals are detected when

they bind to a cellular protein in the cell membrane of the target cell

Transduction• In this stage the signal is converted

into a form that can bring about a specific cellular response• Transduction can occur in a single

step or multiple steps of the signal-transduction pathway.• Molecules involved in the pathway

are often called relay molecules

Response

• Transduced signal triggers a specific cellular response• Cellular response can be anything

from activation of certain genes in the nucleus to catalysis by an enzyme

Signal Reception and the Initiation of Transduction

Signal molecule binds to a receptor protein causing the protein to change shape

• Signal molecules are complementary in shape to a specific site on the receptor and attaches there• Like a lock and key

Signal molecule behaves as a ligand• A ligand is a small molecule that

specifically binds to a larger one• Sometimes causes receptor protein

to change shape• For some, this shape change

activates the receptor

Signal receptors• Most signal are water soluble

molecules that are too large to pass freely through the cell membrane• They must utilize a signal receptor

protein embedded in the cell membrane to transmit information from the extracellular environment to the inside of the cell

3 major types of membrane receptors

• G-protein-linked receptors• Tyrosine-kinase receptors• Ion-channel receptors

G-protein-linked receptors

• Large family of receptor proteins that all have 7 transmembrane helices

G-proteins

• Act as an on/off switch• If GDP (guanosine diphosphate) is

bound – the G-protein is inactive• If GTP (guanosine triphosphate) is

bound – it is active

• G-protein receptor systems are diverse in function• They can be shutdown quickly when

the signal molecule is no longer present

Tyrosine-Kinase Receptors

• These receptors are commonly used for growth factors

• Characterized by having enzymatic activity• Tyrosine kinase is an enzyme that catalyzes

the transfer of phosphate groups from ATP to the amino acid tyrosine on a substrate protein

Different from G-protein receptors

• Binding of a signal molecule does not cause the shape of the receptor to change like in the G-protein receptors• Can activate more than 1 cellular

response (unlike G-proteins)

How it works

• The ligand binding causes phosphate groups from ATP (adenosine triphosphate) to aggregate (combine) with the tyrosines forming a phosphorylated dimer (a protein consisting of 2 polypeptides)

• This aggregation activates the tyrosine-kinase receptor to bind to specific intracellular relay proteins• The activation of the relay proteins

initiates a signal-transduction pathway leading to a variety of specific cellular responses

Ligand gated ion-channel receptors• Protein pores that open or close in

response to a chemical signal• Important in the nervous system

Intracellular Receptors• Some receptors are proteins in the

cytosol or on the nucleus of a cell• In order for these to work, the signal

must be able to pass through the cell membrane

• Many of these signals are hydrophobic so they can pass through easily

• Examples include steroid and thyroid hormones

Protein phosphorylation• Most proteins are activated by

adding one or more phosphate groups to it – this is called phosphorylation• Protein kinase is the enzyme that

commonly transfers phosphate groups from ATP to a protein

Phosphorylation cascade• Protein kinases are often the relay

molecules in the signal-transduction pathway• They often act on each other to

trigger another protein to be phosphylated • This can occur numerous times

until finally triggering the target protein to elicit a cellular response