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k-exam1-10cor - C2006/F2402 10 Key to Exam#1 Corrections...

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C2006/F2402 ’10 Key to Exam #1 Corrections made during the exam are blocked in blue Unless it says otherwise, each answer was worth 2 pts and each explanation 2 pts. 1. PFE1 is a transmembrane protein. This question is about the location and function of PFE1. A-1. PFE1 is best described as (integral) (peripheral on the inside of the cell) (peripheral on the outside) (peripheral, but can’t tell which side) (peripheral or integral). A-2. Given the information so far, (a) PFE1 could be (a ribosomal protein) (clathrin) (a channel protein) (collagen) (tubulin) (none of these). (b). PFE1 could be part of (the basal lamina) (the cytoskeleton) (a desmosome) (a gap junction) (none of these). For each part, circle all reasonable possibilities. Then explain the basic principle very briefly. PFE1 must be embedded in the lipid bilayer. All the proteins and structures in (a) are in the cytoplasm, except collagen and the basal lamina, which are part of the ECM. None are firmly connected to the bilayer. The items chosen in (b) contain transmembrane proteins. (If you thought parts (a) and (b) had to match, and just picked ‘gap junction,’ that was okay if clearly explained.) For this question, 1 pt each correct choice. B. You have a red dye that is a small molecule. There are no transport proteins for this dye in animals. The dye does not cross the intestinal epithelium (IE). If you inject the dye into a cell of the IE, it spreads throughout the epithelium, but it does not enter the interstitial fluid . If you put dye into the lumen, and PFE1 is missing, the dye enters the interstitial fluid. Given this information, where do you expect to find PFE1? B-1. In normal epithelial cells, PFE1 is most likely to be bound to (an integrin) (connexin) (cadherin) (PFE1 from a neighboring cell) (PFE1 in the ECM) (an integrin or cadherin) (any of these). B-2. If PFE1 is missing, should the injected dye spread through the epithelium? (yes) (no) (can’t predict). Explain your reasoning – about how normal PFE1, works and the effects of missing protein. Simplest solution, given the proper corrections: B-1. PFE1 is part of a tight junction, and that PFE1 is the major transmembrane protein that connects the membranes of neighboring cells. (The protein in tight junctions is not cadherin. Cadherins are restricted to adhesive junctions.) Tight junctions, not adhesive junctions, form a seal between the cells of the IE so liquid cannot pass across it. When PFE1 is defective, the tight junctions become leaky. Then liquid containing dye can pass between the cells of the epithelial layer and reach the interstitial fluid. B-2. The dye spreads within the IE through gap junctions. Gap junctions and subsequent spread of the injected dye should be unaffected by changes in tight junctions. An alternative solution is that PFE1 is part of a gap junction, and is bound to (or is) connexin. (In this case it could also be bound to PFE 1from a neighboring cell = connexin from a neighboring cell). When PRE1 is missing, the injected dye leaks out of the cells into the interstitial fluid and the dye does not spread (or does not spread as well) from cell to cell. This answer does not explain how dye added to the lumen could reach the interstitial fluid if PFE1
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