The relative resistance of the tight junctions could

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the relative resistance of the tight junctions could reflect the specific claudins used in their construction, rather than the number of strands. This is an area of ongoing research. References: Claude P (1978) Morphological factors influencing transepithe- lial permeability: a model for the resitance of the zonula occludens. J. Memb. Biol. 39, 219–232. Van Itallie CM & Anderson JM (2006) Claudins and epithelial paracellular transport. Annu. Rev. Physiol. 68, 403–429. A442 Chapter 19: Cell Junctions, Cell Adhesion, and the Extracellular Matrix relative resistance 0 2 4 6 8 mean number of strands 10 0 10 1 10 2 10 3 10 –1 Figure 19–25 Plot of relative electrical resistance versus the number of strands in the tight junction ( Answer 19–33 ).
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DATA HANDLING 19–34 A. Even though all of the claudin-4 has disappeared, the cells still express claudin-1, which is not affected by the toxin. Using antibodies specific for claudin-1, the authors showed that it remained intact at the sites of the tight junctions in the presence of the toxin. B. There are several possible ways that binding of the toxin might cause the claudin-4 strands to disintegrate. Bound toxin could alter the conformation of claudin-4, weakening its affinity for other claudin-4 molecules in the same strand. Alternatively, toxin binding might alter the lateral affinity between claudin-4 molecules in the paired strands from different cells. Finally, the toxin might bind preferentially to claudin-4 monomers, shifting the equilib- rium toward the dissociated form. Since claudin-4 actually disappears when the cells are treated with toxin, it must also be true—regardless of which pos- sibility is correct—that claudin-4 molecules that are outside of paired strands are degraded fairly quickly. C. Because the tight junction prevents molecules from penetrating the junction, added toxin will have access to only one side of the junction. Its inability to work from the apical side suggests that its binding sites on the claudin-4 molecules are accessible only from the basolateral side. If the toxin binds to monomers, as suggested above, then it could be that the monomers are deliv- ered to the basolateral membrane domain, and therefore accessible only from that side of the epithelial sheet. Alternatively, if the strands of claudin molecules are all oriented in the same way—that is, with their ‘top’ surfaces all facing the apical side and their ‘bottom’ surfaces all facing the basolateral side, as would be expected from symmetry principles—then a toxin binding site on the ‘bottom’ surface would only be accessible from the basolateral side. Reference: Sonoda N, Furuse M, Sasaki H, Yonemura S, Katahira J, Horiguchi Y & Tsukita S (1999) Clostridium perfringens enterotoxin fragment removes specific claudins from tight junction strands: evidence for direct involve- ment of claudins in tight junction barrier. J. Cell Biol. 147, 195–204.
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