Courtesy of Kaspar Locher
 B12 TRANSPORTER EXPOSED: Ribbon diagram of the B12 transporter BtuCD from E. coli, with cyclotetravanadate bound at the ATP-binding sites. The transporter is assembled from four subunits, two each of the membrane spanning BtuC (yellow and red) and the cytoplasmic BtuD (green and blue).

The ATP binding cassette transporters epitomize nature's ability to re-use a successful protein motif. With diverse membrane-spanning regions, but highly conserved soluble ATP binding domains, the ABC transporters serve a wide variety of functions and are defunct in disorders such as Tangier disease and cystic fibrosis (see All in the Superfamily). Small wonder then, that the first high-resolution structures of this largest family of transporters were greeted with great acclaim. But there was puzzlement as well; the structures did not agree. "The initial MsbA structure came as a surprise ... although it was undoubtedly the result of an impressive tour de...

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