Term 2 Lecture 14: What Are Downstream Results Of Translocation? Flashcards
Cells show varied responses to combinations of signalling molecules
Cells of multicellular organisms can be exposed to tens to hundreds of signalling molecules in millions of combinations.
The response of a given cell to its environment depends upon this combination (combination signaling)
Some signals e.g. growth factors are required for a cell to survive, some signals instruct a cell to proliferate while others instruct the cell to differentiate into another cell type.
Defects in proliferation and differentiation signalling lead to cancer.
In the absence of any signals the cell dies (apoptosis.) The exact response of any cell depends on the combination of signals it recieves
Signal transduction
Concept first appeared in biological literature in 1997
Biophy.struct.mech. 3:69-77
It’s first use in titles in bio literature was in 1979
Nature 280: 279-284
J. Bacteriol. 138 739-747
Triggered for widespread use in 1980 was associated with Nature 284:17-22
By the year 2000 12% of papers using the word cell also use the phrase signal transduction.
For life sciences signal transduction is the conversion of biotic or abiotic (chemical) information, often outside the cell, into a form inside the cell that can be interpreted and acted upon (reacted to)
Why is signal transduction so important?
Abiotic information includes temperature, pH, light, salinity and hydration. The external environment is always changing and without the ability to detect and respond to the abiotic environment organisms will die.
Biotic information includes hormones, growth factors and neurotransmitters collectively known as extracellular signalling molecules.
The survival of a multicellular organism depends upon the ability of each cell to determine its position and function within the organism.
This is achieved by “social control” where cells signal their status to other cells. Cell behaviour depends upon the signals received.
Microorganisms such as bacteria and yeast are known to communicate inter specially although we currently do not know the mechanisms.
Conclusion: all life on earth is dependent on signal transduction. When signal transduction is aberrant life suffers e.g. cancer
What is the range of cells that can communicate with one another through the use of biological molecules and signal transduction?
Prokaryotes- poorly understood, particularly the range and types of biomolecules that are used to permit communication between organisms in different microbial communities.
E.g. quorum sensing in Psuedomonas aeruginosa allows a coordinated microbial virulence response to the hosts immune response.
Lower eukaryotes: well studied are the role of mating factors in saccharomyces cerevisae (yeast) that signal cells of the opposite mating type to stop proliferating in preparation for sexual mating. Also well studied is the role of 3’5’ cAMP in the starvation response of social amoeba Dictyostelium discoideum . Response is to aggregate to form a stalk and fruiting body with spores which are resistant to environmental stress
Higher eukaryotes: very well studied but our knowledge is still far from complete. In describing general principles of cell signalling we will come across many examples.
Overview of cell signalling
Extracellular signalling molecules:
Synthesised in cells, packaged into secretory vesicles and secreted by specialised signalling cells e.g. neurons within multicellular organisms.
Signal: produces a specific response only in target cells expressing receptor proteins that bind the signal.
Signals are ligands that bind to a protein receptor (on a cell membrane)
Steps of hydrophobic and hydrophilic signalling
See diagram start of notebook 3
Hydrophobic signals
1) diffuse through plasma membrane
2) bind to cytosolic receptors
3) receptor-signal complex moves into the nucleus and binds transcriptional control regions in DNA to activate or repress gene expression
Hydrophilic signalling
1) bind to a specific cell surface receptor - triggering receptor confirmational change and activating it
2) activated receptor then activates one or more downstream signal transduction proteins (e.g. phosphorylation) or small molecule second messengers (e.g. Ca²+)
3) signal transduction proteins or small molecule second messengers activate effector proteins
4a) effector stimulates modification of specific cytosolic proteins ( e.g. phosphorylation or Ca²+ binding) causing shor term changes (seconds to minutes) in cellular function, metabolism or movement
4b) OR: effector moves into nucleus triggering long-term (hours to permanent) changes in gene expression by activating or repressing transcription factors.
5) negative feedback/feedback repression from intracellular signalling molecules
6) destruction of extracellular signal
5 and 6 terminate or downmodulate cellular response
Signalling molecules can act locally or at a distance
A) endocrine: coordinates cell behaviour over long distances. Specialised cells secrete ligands called hormones into the blood stream (e.g. adrenaline) or sap (e.g. auxin) to be distributed around the body. Slow signalling dilute at receptor
B) paracrine: acts over small distances. Diffusion of ligands limited by ECM and enzymes e.g. WNT, hedgehog and BMP proteins in cell fate determination in development
C) Autocrine: the cell produces a ligand that binds to its own receptors e.g. can reinforce a developmental decision amongst cells. The mechanism is strongest among groups of cells enabling that group to enter a specific development pathway e.g. insulin-like growth factors in muscle development. Eicosanoids are fatty acid derivatives made by all cells in all mammalian tissues. On tissue damage eicosanoid production increases and acts in an autocrine fashion to mediate pain, fever and inflammatory response.
D) contact dependent: signalling of integral membrane proteins to adjacent cells
There are ~12 classes of receptor in eukaryotes
Ligand-receptor interaction is specific. A receptor binds to a single type of ligand or class of closely related ligands. The bond involves multiple weak non-cov forces e.g. Van Der Vaals and hydrophobic interactions. Binding causes confirmational changes in the receptor.
The ligand receptor also recruits a second receptor molecule - the receptor is then activated.
This initiates a series of reactions that lead to an event in the cell. This is almost always mediated through protein phosphorylation.
Regulation of protein activity by a kinase/phosphatase switch
This occurs in higher eukaryotes.
Cell surface receptor signalling:
Involves kinase phosphorylation and phosphatase dephosphorylation to regulate target protein activity.
Protein kinase: transfers phosphate from ATP to specific Ser/Thr or Tyr-OH (this phosphorylated residue is part of a specific kinase motif)
Protein phosphatase: hydrolyses Pi off the protein restoring Ser/Thr or Tyr-OH
Protein kinases and phosphates are:
-regulated by signalling processes
- modifiers of specific protein targets containing target motifs
Effect of phosphorylation (and reversal by dephosphorylation) on protein activation/deactivation
It is protein specific
Humans have ~600 types of protein kinase and produce 100 protein phosphatases
The protein phosphatases hydrolyse the Pi group off the phosphorylated target protein to activate it or deactivate it
Phosphorylation can activate or deactivate a protein depending on its type.
Process:
Protein kinase converts an ATP→ADP+Pi to phosphorylate the target protein
Protein phosphatase removes the Pi group dephosphorylating it
Other regulation in signal transduction
Many cellular processes utilise the GTPase super family of proteins (that hydrolyse GTP removing the phosphate)
They are found in all prokaryote and eukaryote cells and are involved in many ‘core’ aka ‘ancient’ processes such as protein elongation (e.g. EF1 alpha or EF2)
GTP binding proteins are used as on/off switches in signal transduction
On/active when GTP is bound
Off to on promoted by GEFs (guanine nucleotide exchange factors.) GEFs catalyse dissociation of bound GDP and replacement by GTP
- NOT phosphorylation of GDP
On to off
GTPase activity GTP→GDP+Pi (on to off)
Accelerated by GAPs (GTPase activating proteins) and RGSs (regulators of G protein signalling)
Process:
1)Inactive G protein with bound GDP
2)GEF removed GDP replacing it with GTP
3) results in active G protein with bound GTP
4) GAPs and RGSs remove a Pi group ( return to 1)
GTPase switch proteins are of 2 large signalling classes
Heterotrimeric G proteins
Activated by direct interaction with surface receptors (GEFs)
Monomeric G proteins
Activated by GEFs that are activated by surface receptors or other proteins
3 major classes of cell signalling
1) G protein linked
2) Enzyme linked receptor
Enzymatic function promoted by a ligand binding to a receptor
3) signalling dependent on multicomplex disassembly
Ligand binds to a complex and it disassembles promoting signal transduction
Two factors determine the response of a cell to a combination of signalling molecules
1) the sunset of receptors that the cell possesses to detect these signals
2) the nature of the intracellular machinery by which the cell interprets the signal
An identical receptor on different cells with different internal signalling machinery will give a distinctly different response to the same signal in different cells
Example of different cells showing different responses to the same signal
Acetylcholine
Causes heart muscle cells to decrease rate and force of contraction
Causes salivary glands to secrete
Causes skeletal muscles to contract
Heart and salivary cells have the same Ach receptors their response is different due to the different intracellular response of salivary gland cells compared to heart cells
And to the difference in acetylcholine receptors on heart and skeletal muscle cells
(Dale and Loewi Nobel prize 1936)
