Cell Bio 11 Flashcards
Signal Transduction
Conversion of 1 Signal into another
A signal from outside of the cell converted into a signal inside the cell
Involves growth factors, cytokines, hormones, ECM, neutrotransmitter, light, sound.
Controls all aspects of normal development and physiology
Initiator of dseases
Key Players in Signal Transduction
Receptor Tyrosine Kinases (RTKs)
G-protein coupled receptors (GPCRs)
Proto-oncogenes
Mitogen activated protein Kinases (MAPKs)
Basic elements of cell signaling
Signal or signaling molecule (ligand; primary messengers)
Receptors:
Cell-surface receptors
Intracellular receptors
Intracellular signaling and effector proteins:
G-proteins, protein kinases + phosphotases
Secondary messengers:
Ca++, cAMP, cGMP, IP3, DAG, NO
Signal or Signaling molecule
Small molecules, epinephrine, acetylcholine, steroids.
Large molecules, growth factors, cytokines.
Cell Signaling: Signaling molecule types
Different types of signaling molecules.
Some are hydrophobic and cross the plasma membrane, binding to receptors in the cell, the complex can move into the nucleus.
Hydrophilic molecules bind to cell surface receptor can effect changes in intermediate proteins.
Nuclear-receptor Superfamily
Lipid soluble hormones bind to intracellular receptores which constitute the NRS of transcription factors.
Intracellular signaling receptors are directly activated by hydrophobic ligands, All receptors have a ligand binding domain and DNA binding domain
Gene activation by a nuclear receptor
Inhibitors binds to receptors from entering the nucleus when glucocorticoid binds it removes the inhibitor allowing GR to enter the nucleus and bind to regulatory sequences in the DNA, the activation domain recruits transcription factors that increase transcription.
Signal Transducton
Hydrophilic proteins have to transmit their signal through an intermediate protein (cell surface receptor), it causes a conformational change in the receptor and then causes the recruitment of other intracellular proteins that transmit signal to effector proteins
Four Forms of intercellular signaling
Endrocine signaling
Paracrine signaling
Autocrine signaling
Signaling by plasma membrane-attached proteins
Endocrine Signaling
Hormone secretion into blood by endocrine gland and target distant cells.
Tissues produce a ligand that enters into the bloodstream and acts distantly on target cells.
Paracrine Signaling
One cell secreting a ligand that acts on an adjacent target cell.
Autocrine Signaling
Target sites on the same cell
Cell producing a ligand that is detected by receptors on the cell surface, autosignaling.
Signaling by plasma membrane attached proteins
They can act locally or at a distance
Signaling by Cell-Surface Receptors (steps)
1- Synthesis and release the signaling molecule by the signaling cell
2- Transport and binding the signal to a specific receptor
3- Initiation of one or more intracellular signal-transduction pathways
4- short term cellular response
5- long-term cellular responses
6- termination of cellular response
Receptor can transmit a signal very quickly by modifying intracellular proteins in close proximity to the receptor affecting metabolism, function or movement.
Rapid change in cell behaviour
Altered protein function
i.e. changes in ion transport, cell movement, secretion or metabolism
Slow change in cell behaviour
A number of steps to activate transcription factors that alter gene expression.
i.e. gene regulated changes in cell growth and division
Animal cell’s dependence on multiple extracellular signal molecules
A cell is bombarded by different signals.
Mitogenic factors: activate cell cycle
Stem cell differentiation in response to signals
Apoptogenic factors or complete absence of trophic factors, absence of survival factors might lead to the cell developing an apoptotic response.
Ligand-receptor interactions
Binding specificity is based on the molecular complementarity between the interacting surface of a receptor (binding interface) + ligand (noncovalent forces)
It triggers a conformational change in the receptor
very often signaling molecules (ligands) induce receptor dimerization.
An extracellular protein has a part that sticks outside the cell and it can interact with the ligand, molecular complementarity. Structure of the ligand and its ability to bind to it, complementary structure, and charge
The dissociation constant
R+L -> RL
at equilibrium we have a simple equilibrium binding equation:
Kd = [R] [L]/[RL]
Kd is the measure of affinity of a receptor to its ligand
Kd is the ligand concentration required to bind 50% of the cell surface receptors
The lower the Kd the higher the affinity of the ligand for the receptor
Functional expression assay to identify a cDNA encoding a cell surface receptor
If you have a transmembrane receptor it is difficult to purify it due to the hydrophobic domain, the hydrophobic domain causes the protein to fold up together.
Cultured cells do not express receptor for ligand X.
Transfect cells with cDNA library and screen for cellular phenotype associated with ligand X.
Identify incorporated cDNA by PCR followed by sequencing
The receptor protein can be deduced from the cDNA sequence
Can perform further studies by mutating specific amino acids to determine essential ligand binding domain
Regulation of protein activity by a kinase/phosphatase switch
Tyrosine kinases/Serine/Threonine kinases or Phosphatases
When a ligand and receptor interact that release a signal related to changes in phosphorylation of intracellular proteins, certain kinases can be activated by these signaling molecules, which adds a phosphate group to specific amino acids within a substrate protein.
It can lead to activation or inactivation.
Protein phosphatases can remove phosphate to turn the protein off.
G-proteins exist in 2 forms
Active-bound to GTP
Inactive-bound to GDP
Activation of G-protein is triggered by
a signal
hormone binding to the receptor
Helped by Guanine nucleotide exchange factor.
leads to GTP binding and activating the G-protein
Gly-60 and Thr-35 residues bind to the phosphate attached to GTP
