Cell Communication - Signal Reception

Ch. 11

Section 11.2

Whiteboards please

How cells “hear” the call

• The target cell has a receptor protein that recognizes the chemical signal Recognition happens when

the signal molecule binds to the receptor

They fit together like lock and key

Critical Vocab!!

• Signal molecule = ligand

Signal Reception

• Ligand binds to receptor • Receptor protein changes shape• This starts the transduction pathway• Leads to cell response

Quick Think

•How do cells “hear” the message from other cells?

Chemical Signals

• Most ligands (chemical signals) are too large to pass through the cell membrane

• BUT…

Too big

Intracellular receptor proteins

• Some signal receptors are inside the target cell Dissolved in the cytosol In the nucleus

• These chemical signals have to be hydrophobic or small enough to cross the membrane

• Examples are steroid & thyroid (lipids)

hormones in animals Nitric oxide (NO) - small gas

Testosterone - a steroid signal molecule

• Testosterone is a hormone, a steroid, secreted by the testis

• It travels through the blood stream• It enters all kinds of cells• But the only cells that can “hear” it

are the target cells that have the right receptor molecule inside the cell

• Once inside, testosterone binds to the receptor and helps turn on genes that control male sex characteristics

How do genes get “turned on”?

• Protein receptors that “turn on” genes are called transcription factors

• Transcription factors control which genes are “turned on” or transcribed into mRNA

Quick Think

• Where are intracellular reception proteins?

• Why can their chemical signals pass into the cell?

Quick Think

• Nerve Growth Factor (NGF) is a water-soluble signal molecule. Would you expect the receptor for NGF to be intracellular or in the cell membrane? Why?

Most signal receptor proteins are in the cell membrane

• There are 3 main types of cell membrane signal receptors G-protein-linked

receptors Receptor tyrosine

kinases Ion-channel receptors

G-protein linked receptors

• Made up of a receptor protein + a G protein

• They have 7 helices

The G-protein part

• The G-protein part is like an on/off switch for the receptor protein

• When GDP (cousin of ATP) is bound to it, the G

protein is inactive

• Ligand attaches• G-protein binds to

GTP and becomes active

• G protein slides down cell membrane

• G protein binds to an enzyme to start transduction

The G-protein

• G-protein receptor systems are really common and play a role in lots of different cell processes Development of embryos Vision & Smell

• Lots of diseases involve G-proteins Bacterial infection like botulism

and cholera interrupt G-protein function

Most drugs (medicines) work by influencing a G-protein pathway

Quick Think

•How does a G-protein linked receptor function?

Tyrosine-kinase receptors

• Best when the cell needs to trigger several cell responses at once

Tyrosine-kinase receptors

• kinase - enzyme that catalyzes the transfer of phosphate groups

• So it is an enzyme that functions to transfer a phosphate group from ATP to tyrosine

Tyrosine-kinase receptors

• Has 3 parts: Extracellular (outside

the cell) signal-binding site

1 helix An intracellular

(inside the cell) tail with a bunch of tyrosines on it

Tyrosine-kinase receptors

• Once 2 signal molecules binds to 2 tyrosine-kinase receptors, they pair up to form a dimer

Tyrosine-kinase receptors

• Once this happens, this causes the tyrosine-kinase part of the receptor to take phosphate from ATP and add it to the tyrosine tail

• This makes the receptor protein “fully activated”

Tyrosine-kinase receptors

• Once the tyrosine-kinase receptor is fully activated, it is recognized by other proteins in the cell

• Each tyrosine-kinase receptor may activate many different proteins that may lead to many different cell responses

Quick Think

•Summarize how a tyrosine-kinase receptor functions.

Ligand-gated ion channel protein

• Receptor protein that acts as a gate for entry into the cell

• When a signal molecule binds to the receptor protein, the gate opens to allow specific ions (like sodium or calcium) to flow through it

Ligand-ion-gated channel protein

• When the ligand (the signal molecule) binds, it changes the shape of the channel protein

• This causes the “gate” to open and allow ions through

• When the ligand dissociates, the gate closes

Ligand-gated-ion channel protein

• Important in the nervous system Neurotransmitter binds as a

ligand Ion-channel proteins open Ions flow into cell Triggers an electrical signal

that moves down the length of the cell

Quick Think

•Summarize how a Ligand-gated ion channel functions.