Are most transporters and channels beta barrels?
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Given the sequence of transporters or channels of unknown secondary structure, it is usual to predict their putative transmembrane regions as α-helical. However, recent evidence for a facilitative glucose transporter (GLUT1_ appears inconsistent with such predictions, which has led us to propose an alternative folding model for GLUTs based on the 16-stranded antiparallel β-barrel of porins. Here we apply the same predictive algorithms we used for GLUTs to several other membrane proteins. For some of them, a high-resolution structure has been derived (β-barrels: Rhodobacter capsulatus andEscherichia coli porins; multihelical: colicin A, bacteriorhodopsin, and reaction center L chain); we use them to test the prediction procedures. The other proteins we analyze (GLUT1, CHIP28, acetylcholine receptor alpha subunit, lac permease, Na+-glucose cotransporter, shaker K+ channel, sarcoplasmic reticulum Ca2+-ATPase) are representative of classes of similar membrane proteins. As with GLUTs, we find that the predicted transmembrane segments of these proteins are consistently shorter than expected for transmembrane spanning α-helices, but are of the correct length and number for the proteins to fold instead as porin-like β-barrels.
Key wordsmembrane protein structure transporters channels porins beta barrels
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