7.1 cell signaling principles

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what are the 3 stages of cell signaling

1. reception

2. transduction

3. response

what happens during the reception stage

the signal (which is a ligand) binds to a receptor

what happens during the transduction stage

the message is transmitted through a series of intracellular events

what happens during the response stage

the cell responds to the signal in some way, such as:

• gene expression

• cell growth

• apoptosis

what are receptors

specialised proteins that bind specifically to signaling molecules

what are the 2 main classes of ligands

• hydrophilic ligands

  • cannot cross the plasma membrane

  • bind cell-surface receptors

• hydrophobic ligands

  • can cross the plasma membrane

  • bind intracellular receptors

what are the different modes of cell signaling

• distance

• speed

what is binding

binding initiates a conformational change that triggers a chain of intracellular events and subsequent cellular response

what is special about the receptors and the ligands

receptor is highly specific for its ligand

• receptor binds its ligand with high affinity and selectivity

• cells can be more or less sensitive to a signal depending on receptor number

• receptor specificity is the basis of targeted drug therapy

what does it mean when cellular response can be fast or slow

fast:

• affects the activity of proteins such as enzymes

slow:

• requires changes in gene expression and the synthesis of proteins

what are the different types of cell signaling

1. direct signaling

2. paracrine signaling

3. endocrine signaling

what is direct signaling

adjacent cells interact directly through physical contact or specialised junctions

what are the different types of direct signaling

1. juxtacrine signaling

• requires cells to be physically touching

• usually a protein fixed on the surface of one cell interacting with a receptor on the surface of the adjacent cell

2. through gap junctions

• small molecules pass directly from one cell’s cytoplasm to the next without entering extracellular fluid

• occurs between physically connected cells

what is paracrine signaling

cell produces signals to induce changes in nearby cells, rather than acting on itself or distant cells

what are the different types of paracrine signaling

1. autocrine signaling

• signaling cell acts as both the sender and the target cell

• they secrete their own ligands and binds back to their own

2. neuronal signaling

• synaptic vesicle releases neurotransmitters to the target cell

what is endocrine signaling

hormones are secreted into the bloodstream to regulate distant target cells

• endocrine signaling cells only produce hormones

how do intracellular proteins act as molecular ON/OFF switches

by being activated / inhibited by addition or removal of phosphate group

• phosphorylated: ‘on’, dephosphorylated: ‘off’

• GTP-bound: ‘on, GDP-bound: ‘off’

what is a protein kinase

a type of enzyme that transfers a phosphate group from high-energy donor molecules to specific substrates

what is the process of a cell signal to the entire system

signal amplification, integration and feedback

what is signal termination

when signals must be actively terminated, otherwise a single binding event would trigger a permanent response

what are the signal termination mechanisms

• ligand degradation or removal

• receptor internalisation

• dephosphorylation by phosphates

• intrinsic GTPase activity of G proteins

• breakdown of intermediates

what is the molecule that regulates various responses

the neurotransmitter, acetylcholine