12.1 Signaling IV Flashcards
Which pathways require regulated proteolysis?
1) Notch and Delta
2) Wnt / b-catenin
3) Hedgehog
4) NFk-B
Signal pathways that use degradation of signaling components or inhibitors of signaling components are involved in what type of functions?
many of these of involved in early development where rapid changes in gene expression are produced by the degradation of proteins that inhibit the function of key transcription factors. q
The notch receptor and its ligand Delta play a major role in what processes?
in development processes of most tissues.
Delta signaling is an example of what type of signaling event?
contact dependent as it is used by one cell to tell its neighboring cell what to do.
How do drosophila maintain the correct proportion of neuronal and epithelial cells?
-using the notch/delta signaling system
- As individual nerve cells begins to develop, it expresses delta on its surface.
- neighboring cells are signaled through the notch receptor that their neighbor has become a neuron and this prevents them from also becoming neurons.
What happens if there is a mutation in either delta or notch?
too many cells will become neurons and not enough will become epithelial cells.
In the notch/delta signaling, which protein undergoes proteolysis?
The notch receptor undergoes proteolysis in the signaling system.
-in undergoes proteolysis in three different places and only two are dependent on delta signaling.
What are the stages of the notch receptor undergoing proteolysis?
1) Interaction between notch and delta causes cleavage of the protein by an extracellular protease followed by an intracellular one.
2) the cleavage event on the inside of the cell membrane is accomplished by a proteolytic enzyme complex called gamma secretase. . One the regulatory subunits, called presensilin, is mutated in some forms of
early Alzheimer’s disease
3) Notch tail moves into nucleus to activate Notch-responsive genes by converting a transcriptional repressor to an activator transcription factor (CSL).
4) CSL is converted from an inactivator of transcription factor to an activator of transcription.
What is unique about the notch receptor?
Unlike most receptors, notch cannot be recycled. Once it is activated by proteolysis you can’t rejoin the pieces back together to form the inactive form.
What is Wnt? And what type of events/processes is it involved in?
a signaling protein that is secreted by cells and detected by other cells having an appropriate receptor and regulates proteolysis of beta catenin
INVOLVED in different developmentally regulated events
What is the basic mechanism of Wnt signaling?
to stabilize a transcription enhancer called beta catenin
In the absence of Wnt signals, what happens to beta catenin?
beta catenin is phosphorylated by a degradation complex and the phosphorylated beta catenin is target for proteolytic degradation by the general machinery for protein degradation and recycling in cells.
How do the receptors for Wnt look in drosophila?
they are frizzeld and the LDL receptor related protein or LRP which binds to a fatty acid linked to Wnt signaling peptide.
What is frizzled in drosophila?
a seven pass transmembrane protein that is not an actual G protein. instead the binding of Wnt to frizzled and LRP causes the recruitment of an intermediate called disheveled and the kinase that are part of the degradation complex. In turn this causes phosphorylation of the protein.
What is Axin in the Wnt signaling?
Axin is a scaffolding component of the degradation compex and after its phosphorylated the degradation complex falls apart. As a consequence, beta catenin is no longer phsophorylated which allows beta catenin to be present long enough to enter the nucleus and regulate the transcription of Wnt responsive genes.
What is unique about the Wnt signaling pathway?
the active signaling component, beta catenin, is always being produced but then it is inactivated by destroying it (unless an appropriate activation signal is present. This allows the cell to respond rapidly to signaling events.
