1935 a in contrast to what is observed for

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19–35 A. In contrast to what is observed for intracellular polymers such as micro- tubules and actin filaments, tight junction strands do not elongate or shorten appreciably on the time scale of observation in these studies. B. Individual strands join end-to-end and break apart, as indicated by the arrows in Figure 19–26A. C. Formation of T-junctions is indicated by the arrows in Figure 19–26B. D. The reviewer makes a good point. In cells that form tight junctions, the plas- ticity of the strands may well be affected by the binding of claudin monomers to scaffolding proteins that contain PDZ domains. Nevertheless, these movies show that claudins can organize themselves into strands that pair with similar strands in apposed cells. And they can form T-junctions with other strands, which resemble those in natural tight junctions. TIGHT JUNCTIONS AND THE ORGANIZATION OF EPITHELIA A443 0 5 10 15 20 25 30 35 40 45 minutes (B) Clip 2 (A) Clip 1 0 5 10 15 20 25 30 35 40 45 50 55 minutes Figure 19–26 Changes in connections between paired claudin strands in apposing plasma membranes ( Answer 19–35 ). (A) Frames from movie clip 1. End-to-end joining of strands and strand breakage are indicated by arrows . (B) Frames from movie clip 2. Formation of T-junctions is indicated by arrows .
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Reference: Sasaki H, Matsui C, Furuse K, Mimori-Kiyosue Y, Furuse M, & Tsukita S (2003) Dynamic behavior of paired claudin strands within appos- ing plasma membranes. Proc. Natl Acad. Sci. USA 100, 3971–3976. 19–36 The key observation in your experiments (which your advisor saw in the data) is that the fluorescent label did not spread across all the apical sur- faces, but remained confined to the infected cells. As shown in Figure 19–10A, an integral feature of the lipid model is that the outer monolayers of the interacting membranes are fused so that they are continuous on the api- cal surfaces of adjacent cells. For the same reason, the outer monolayers of all basolateral surfaces would be continuous. Confinement to individual cells is not consistent with the expectations of the lipid model. Why, if the protein model is correct, are lipids in the outer monolayer not freely diffusible between the apical and basolateral surfaces of an individual cell? The confinement of lipids to the apical surface is not entirely unex- pected in the protein model. After all, if the protein seals the space between the cells, it must form a fairly impenetrable barrier even at the surface of the membrane. If the barrier actually were buried slightly in the outer mono- layer, as it is likely to be, it would interfere with the diffusion of lipids from one side to the other. Since addition of Ca 2+ allows free diffusion of lipids in the outer monolayer, the continuity of the sealing strands in a tight junction must be disrupted, not only between adjacent cells but also within the membrane of an individual cell.
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  • Spring '04
  • EricLander
  • cell biology, Collagen, basal lamina, cell adhesion, J. Cell Biol.

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