Question

1) You are studying TGF-B signaling in a line of cultured human intestinal epithelial cells by measuring the amount of collagen IV mRNA induced 10 hrs. after TGF-B addition. Normally, TGF-B causes a 5- fold increase in Col IV mRNA. a) You overexpress in this cell a mutant type II TGF-B receptor lacking its cytosolic domain (that is you express it at a level 10-fold higher than the natural (functional) receptor protein). You find that the cells no longer respond to TGF-B addition by inducing significant levels of Col IV mRNA. Explain. b) Suppose you overexpress in this cell a mutant type II TGF-B receptor lacking its exoplasmic domain. Would you expect an effect on TGF-B signaling? Why or why not? c) Suppose you overexpress in this cell a mutant type I TGF-B receptor lacking its cytoslic domain. Would you expect an effect on TGF-B signaling? Why or why not?

Answer 1

• Firstly TGF beta family receptors are serine/threonine kinases.
• If the receptor lacks its cytoplasmic domain it is unable to start the further signaling cascade because the receptor is no more able to phosphorylate the further proteins in the pathway.
• So in the first case it's obvious that the expression of col IV expression decreases in the presence of mutant TGF beta II receptor lacking its cytoplasmic domain.
• In case B somewhat similar is the reason, as receptor lacking its exoplasmic domain it will not binds to its ligand. In the absence of ligand signaling cascade automatically in the terminated state resulting in decreased expression of col IV mRNA.
• In case C mutant TGF beta lacking it's cytoplasmic domain unable to phosphorylate further components of the pathway, unnecessarily terminating it; which results in decreased expression of col IV mRNA.
• For any receptor to function properly it should have both its cytoplasmic domain and extracellular domain intact and functioning, as on acts as ligand-binding site and other functions as the signal transducer.