Glut4 translocation Flashcards
What is the response of the liver to the fed state?
Increase in insulin secretion by liver in response to blood glucose levels. Liver produces more glycogen, decreases gluconeogenesis and increases de novo lipogenesis. In adipose tissue lipolysis is downregulated and in the muscles glucose transport and synthesis is upregulated.
What is Glut4?
Glucose transporter, removes glucose from bloodstream for storage in skeletal muscle and fat
What are the characteristics of Glut4?
- Part of large family of transporters
- 12 transmembrane regions (8 pore lining and 4 structural) with intracellular N and C termini
- On C and N terminal are regulatory regions which control trafficking to membrane
What are the properties of the N and C terminals of Glut4?
N - phenyl alanine
C - di-leucine domain
How is Glut4 stored?
Within Glut4 storage vesicles (GSV)
Basal state - stored away from membrane
Insulin burst - detected by insulin detectors cause vesicles to move to membrane (involves SNARE proteins) and take glucose into the muscle
- GLUT4 translocation
Consistent levels of insulin - constant GLU4 at membrane
What is the structure of the insulin receptor?
- Tyrosine kinase receptor
- 2 alpha extracellular units (exclusively bind insulin) and 2 beta subunits bound to membrane with intracellular tyrosine kinase
- Disulphide bonds link chains
What occurs when insulin binds to an insulin receptor?
- Activation of intracellular kinase cross-phosphorylates intracellular limbs
- Acts as recruitment site for downstream proteins such as IR which can activate PI3 kinase
Which intracellular regions of the insulin receptor are esential for function?
Lysine 1018 - critical for ATP binding (source of phosphate)
3 tyrosine regions
1. Regulatory loop
2. Juxtmembrane region: involved in substrate binding
3. C-terminal: regulate signalling and binding
What is the IRS?
Insulin receptor substrate, acts as scaffold to recruit proteins to the membrane
What is the structure of IRS?
2 isoforms: IRS1 & IRS2
- Has a number of domains to mediate membrane binding: PTB (binds to phosphorylated tyrosines and juctamembrane position), PH (binds to phospholipids, PIP3), remaining has C terminal region with up to 20 phosphorylated tyronsine residues as well as some serine and threonines. These, when phosphorylated, regulate proteins that will interact with IRS
What is PI3K?
Phosphatidyl-inositol 3 kinase (lipid kinase) converts PIP2 to PIP3
2 subunits:
- P110 (catalytic)
- P85 (regulatory)
What is the structure of the P110 catalytic subunit of PI3K?
Contains main kinase domain, a number of domains mediating protein-protein interactions and region binding to P85
What is the structure of the P85 regulatory subunit of PI3K?
- P85 has P110 binding region, a GAP domain (GTPase activating protein) and a number of domains (SH2(bind to phosphorylated tyrosine residues) , SH3 (bind to proline rich regions in other proteins), proline (interact with SH3 on other proteins)) which mediate protein-protein interactions
How does PIP3 activation affect GLU4?
- Activation of Akt (protein kinase B) recruited to membrane
- Phosphorylated on two sites letting it be stable and active and phosphorylate downstream targets
What evidence is there for the role of akt in glucose transport?
- Akt knockout in mice substantially increases blood glucose levels
- Insulin can directly activate akt2
