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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 pg, 21). Small wonder then, that the first high-resolution structures or 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 force," writes Kaspar Locher, of the Swiss Federal Institute of Technology in an E-mail interview.
In the first of this issue's Hot Papers, Geoffrey Chang and Christopher Roth of the Scripps Research Institute, La Jolla, Calif., crystallized a lipid exporter known as MsbA from Escherichia coli. (1) MsbA facilitates the export of lipid A, a critical outer membrane component, across the inner plasma membrane. Since it is essential for cell growth, MsbA is a potential target for new antibiotics.
This issue's second Hot Paper, published several months later, described the structure of the vitamin B12 transporter BtuCD, also from E. coli (2) Locher crystallized the protein while in the laboratory of Chang's former mentor, Douglas Rees, at the California Institute of Technology. BtuCD imports vitamin B2.