Fas ligand is a trimeric, extracellular protein that binds to its receptor, Fas, which

Chapter 0, Problem 18-8

(choose chapter or problem)

Fas ligand is a trimeric, extracellular protein that binds to its receptor, Fas, which is composed of three identical transmembrane subunits (Figure Q183). The binding of Fas ligand alters the conformation of Fas so that it binds an adaptor protein, which then recruits and activates caspase-8, triggering a caspase cascade that leads to cell death. In humans, the autoimmune lymphoproliferative syndrome (ALPS) is associated with dominant mutations in Fas that include point mutations and C-terminal Figure Q181 Appearance of paws in Apaf1/ and Casp9/ newborn mice relative to normal newborn mice ( 186). (From H. Yoshida et al., Cell 94:739 Casp9 750, 1998. With permission from Elsevier.) Apaf1 p18.02/18.02 / +/+ / +/ Figure Q182 Time-lapse video fluorescence microscopic analysis of cytochrome cGFP release from mitochondria of individual cells ( 187). (A) Cells observed for 6 minutes, 10 hours after UV irradiation. (B) Cells observed for 8 minutes, 17 hours after UV irradiation. One cell in (A) and one in (B), each outlined in white, have released their cytochrome cGFP during the time frame of the observation, which is shown as hours:minutes below each panel. (From J.C. Goldstein et al., Nat. Cell Biol. 2:156162, 2000. With permission from Macmillan Publishers Ltd.) p18.04/18.04 (B) (A) 17:10 17:18 10:09 10:15 1034 Chapter 18: Cell Death truncations. In individuals that are heterozygous for such mutations, lymphocytes do not die at their normal rate and accumulate in abnormally large numbers, causing a variety of clinical problems. In contrast to these patients, individuals that are heterozygous for mutations that eliminate Fas expression entirely have no clinical symptoms. A. Assuming that the normal and dominant forms of Fas are expressed to the same level and bind Fas ligand equally, what fraction of FasFas ligand complexes on a lymphocyte from a heterozygous ALPS patient would be expected to be composed entirely of normal Fas subunits? B. In an individual heterozygous for a mutation that eliminates Fas expression, what fraction of FasFas ligand complexes would be expected to be composed entirely of normal Fas subunits? C. Why are the Fas mutations that are associated with ALPS dominant, while those that eliminate expression of Fas are recessive?