GB1- Chapter 11

Cell Communication

local signaling- direct contact

direct contact

cell junctions- allows molecules to pass through without going through plasma membranes

ex) gap junctions in animal cells/ plasmodesmata in plant cells

cell surface molecules- cell surface molecules interact with each other

local signaling- secreted messenger

secreted messenger- travels short distances.

paracrine signaling- animal cells secreting messengers “para”- around

Growth Factors-stimulates nearby target cells to grow and divide

autocrine signaling- cell secretes signaling molecules that bind receptors to its own surface= alters its own behavior

synaptic signaling- an animal nervous system where electric signal releases a neurotransmitter/ used to treat depression, anxiety, PTSD

Long Distance Signaling

Hormones travel to target cells through the circulatory system. Cells can only respond to the hormone signals if they have the correct receptor proteins

Three stages of cell signaling

1. signal reception- cell surface receptor protein detects signal. The binding of a ligand (signal molecule) to the receptor is highly specific. Most receptors are on plasma membrane but some are in nucleus and cytoplasm.

2. signal transduction- The alteration of receptor shape due to the binding of the signal to the receptor is the first step to transduction. This initiates a multi-step signal transduction pathway

3. cellular response- a specific response in the target cell

membrane receptors

receptor proteins have specific sites where most water soluble molecules can transmit information from extracellular to intercellular environment

1. G protein coupled receptors- largest family, all G proteins are all very similar in structure, extremely wide spread and diverse functions

2. receptor tyrosine kinases

3. ion channel receptors

G protein coupled receptors

1. signaling molecule activates receptors

2. G protein is activated by GTP

3. The activated G protein moves and activates an enzyme

4. The activated enzyme produces a cellular response

5. GTPase hydrolyzes GTP into GDP and a phosphate group

• allows pathways to shut down quickly when G protein leaves

Receptor tyrosine kinases (RTKs)

1. signaling molecules attach to receptors. dimerization- monomers come together as dimers

2. dimer’s tyrosine kinase regions are activated- unphosphorylated dimer

3. 6 ATP drops off phosphate groups

4. dimer’s tyrosine kinase regions are fully activated- phosphorylated dimer

5. attracts relay proteins, triggering a transduction pathway that leads to cellular response.

Ligand- gated ion channel

1. signal molecule (ligand) attaches to receptor, changing its shape

2. the ion channel is opened when ligand is binded. specific ions like Na or Ca moves through.

3. ion channel is closed when the ligand falls off

intercellular receptor proteins

found in the nucleus or cytosol. Only small or hydrophobic molecules can cross the membrane and activate receptors. The activated receptors work as a transcription factor that turns genes on and off.

multistep process

can amplify a signal, allows for more detailed coordination and regluation

signal transduction pathways

phosphorylation cascade

1. signal molecule binds to a receptor in the first step

2. activated receptor activates another receptor and so on until the producing response is activated

Protein phosphorylation and dephosphorylation

protein phosphatases- regulates protein activity by turning it on/off

second messengers

small, nonprotein water-soluble molecules that spread throughout the cell through diffusion

• are used in GPCRs and RTKs

Cyclic AMP- second messenger

produced from ATP

• G-Protein coupled receptors activate adenylyl cyclase

• adenylyl cyclase- an enzyme in plasma membrane converts ATP to cAMP in response to extracellular signal

• cAMP activates protein kinase A

Vibrio Cholerae

modifies G protein so that it’s stuck in its active form so that it continually makes cAMP. the constant secretion of cAMP causes large amounts of salt to be released into the intestines. Water is lost through osmosis and the person dies from loss of water and salt.

Calcium ions (Ca2+)- second messenger

calcium ion concentration is much higher outside the cell than inside the cytosol. Any small calcium number change represents a relatively large percentage change in calcium concentration. A signal from the signal transduction pathway may increase calcium in the cytosol.

inositol triphosphate (IP3) and diacylglycerol (DAG) also work as second messengers used in the three signal transduction pathways

cellular response in nucleus or cytoplasm

nucleus- signaling pathways regulate enzyme synthesis by turning genes on/off.

the final activated molecule in the signaling pathway can function as a transcription factor for DNA

signal could cause

opening or closing of an ion channel in the plasma membrane or a change in metabolic enzyme activity

four aspects of signal regulation

amplification of the signal- enzyme cascade amplifies cell response to the signal. the number of activated products can be much greater than in the previous step.

specificity of the response- different cells have different proteins that allow for detection and response to different signals. The same signal can have different effects in cells with different proteins and pathways.

efficiency of response- scaffold proteins- large relay proteins to which several other relay proteins are attached. increases efficiency by grouping different proteins in the same pathway.

termination of the signal- low concentration of signaling molecules would only bind to few receptors. The unbound receptors revert to their old state.

apoptosis

Programmed cell death for infected, damaged, or cells that are at the end of their functional lives.

cell components are chopped up and packaged into vesicles that are digested by scavenger cells. This is to prevent dying enzymes from leaking out and damaging neighboring cells

ex) a normal part of hand and foot development

• relations with Parkinson’s and Alzheimer’s as well as some cancers