1962 false of the 24 or so different kinds of

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19–62 False. Of the 24 or so different kinds of integrins in humans, all but one are linked to actin filaments. The remaining variety connects to the intermedi- ate filament network formed by keratin. 19–63 False. Integrins are dynamic molecules that fold to hide their binding sites in the absence of strong intracellular or extracellular ligands. THOUGHT PROBLEMS 19–64 The high level of activation when alanine was substituted for D723 in the b chain, or for R995 in the a chain, indicates that those residues are somehow important for holding the a IIb b 3 integrin in an inactive state. The ‘charge- swap’ experiment, which showed that D723R paired with R995D was as inactive as the wild type, suggests strongly that these two residues form an electrostatic attraction—a salt bridge—that helps to hold a IIb b 3 integrin in its inactive configuration. It follows that inside-out signaling is probably triggered by breaking this salt bridge. Reference: Hughes PE, Diaz-Gonzalez F, Leong L, Wu C, McDonald JA, Shat- til SJ & Ginsberg MH (1996) Breaking the integrin hinge: A defined structural constraint regulates integrin signaling. J. Biol. Chem. 271, 6571–6574. A448 Chapter 19: Cell Junctions, Cell Adhesion, and the Extracellular Matrix
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CALCULATIONS 19–65 Assuming that the area of a platelet can be approximated as the areas of two circles, each 2 m m in diameter, the surface area of a platelet is 2 ¥ p r 2 = 6.3 m m 2 , which is 6.3 ¥ 10 6 nm 2 . At 80,000 integrins per platelet, each integrin occupies 78.8 nm 2 (6.3 ¥ 10 6 nm 2 /80,000 integrins). Assuming that each integrin is 10 nm in diameter, the cross-sectional area of an integrin is 78.5 nm 2 ( p ¥ 5 2 ). With the assumptions stated in this problem, then, integrins would be cheek by jowl in the membranes of platelets. Regardless of the spe- cific assumptions, integrins clearly occupy a large fraction of the surface area of platelets, as befits their critical role in platelet function. DATA HANDLING 19–66 A. Several of the experiments in Table 19–5 indicate that the stimulated bind- ing due to the anti- b 1 antibody occurs by the interaction of fibronectin with a 5 b 1 integrin. Antibody against the a 5 chain blocks fibronectin binding in the presence or absence of the anti- b 1 antibody. Anti-fibronectin antibody interferes with anti- b 1 stimulated binding. And most conclusively, a peptide containing the RGD motif (through which integrins bind fibronectin) reduces the stimulated binding, whereas a similar peptide without the com- plete RGD motif has no effect. B. Synthesis of new integrin molecules or transfer from an intracellular com- partment would require ATP. Experiments showing that metabolic poisons have no effect on anti- b 1-stimulated binding eliminate these possibilities. Thus, the increased affinity of a 5 b 1 integrin for fibronectin seems to be due to some effect of the anti- b 1 antibody on the existing population of integrins.
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  • Spring '04
  • EricLander
  • cell biology, Collagen, basal lamina, cell adhesion, J. Cell Biol.

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