The string of 20 hydrophobic residues beginning with

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Unformatted text preview: bilayer 5. The string of 20 hydrophobic residues beginning with L2 likely forms an ahelix that spans the phospholipid bilayer. R22 and H23, if positively charged, probably would interact favorably with the negatively charged phosphate-ester groups of the phospholipids. (Integral membrane proteins often also have Trp residues at the membrane surface. This protein has Trps at positions 24 and 26.) 6. One way to measure lateral diffusion of a membrane protein or phospholipid is to label the molecule with a fluorescent dye that can be bleached (made nonfluorescent) by focusing a flash of light from a laser to a small spot in the membrane. Fluorescence from the bleached region then is measured as a function of time through the same microscope that was used to focus the laser. The fluorescence drops abruptly at the time of the flash, and then grows back to its original level as the bleached molecules diffuse out of the spot and unbleached molecules diffuse in. (See the text pp. 383-384.) In classical experiments, Edidin and Frye labeled membrane proteins in one preparation of eukaryotic cells with a fluorescent dye that emitted red light, and proteins in another preparation with a dye that fluores...
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