Becker's World of the Cell 9th Edition by Lewis Kleinsmith, Jeff Hardin, Gregory Paul Bertoni

Exercise 6

Under stress. The following questions focus on stress fiber contractility. (Before answering (b)and (c), you may wish to refer to Chapter 13 for a refresher on the protein Rho.)
(a)Stress fibers of nonmuscle cells contain contractile bundles oi actin and myosin II. For stress fibers to contract or develop tension, how would actin and myosin have to be oriented within the stress fibers
(b)Fibroblasts can be placed on thin sheets of silicone rubber. Under normal circumstances, fibroblasts exert sufficient tension on the rubber so that it visibly wrinkles. Explain how the ability of fibroblasts to wrinkle rubber would change if the cells were treated with a permeable from of C3 transferase, a toxin (from the bacterium Clostridium botulinum that covalently modifies an amino acid in Rho. The modification renders Rho unable to blind to proteins that it would normally bind to when active.
(c)The experiment in (b)is repeated, but instead constitutively active Rho is introduced into the fibroblasts.