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J Biol Chem, Vol. 274, Issue 35, 25159-25166, August 27, 1999

Properties of a Cyclodextrin-specific, Unusual Porin from Klebsiella oxytoca

Markus Pajatsch, Christian Andersen^§, Anton Mathes^¶, August Böck, Roland Benz^§, and Harald Engelhardt^¶

From the  Institute of Genetics and Microbiology, University of Munich, Maria-Ward-Strasse 1a, D-80638 Munich, ^§ Lehrstuhl für Biotechnologie, Biozentrum, Am Hubland, University of Würzburg, D-97074 Würzburg, and the ^¶ Max-Planck-Institut für Biochemie, Abteilung Molekulare Strukturbiologie, Am Klopferspitz 18a, D-82152 Martinsried, Germany

The function of CymA, 1 of the 10 gene products involved in cyclodextrin uptake and metabolism by Klebsiella oxytoca, was characterized. CymA is essential for growth on cyclodextrins, but it can also complement the deficiency of a lamB (maltoporin) mutant of Escherichia coli for growth on linear maltodextrins, indicating that both cyclic and linear oligosaccharides are accepted as substrates. CymA was overproduced in E. coli and purified to apparent homogeneity. CymA is a component of the outer membrane, is processed from a signal peptide-containing precursor, and possesses a high content of antiparallel -sheet. Incorporation of CymA into lipid bilayers and conductance measurements revealed that it forms ion-permeable channels, which exhibit a substantial current noise. CymA-induced membrane conductance decreased considerably upon addition of -cyclodextrin. Titration experiments allowed the calculation of a half-saturation constant, K_S, of 28 µM for its binding to CymA. CymA assembled in vitro to two-dimensionally crystalline tubular membranes, which, on electron microscopy, are characterized by a p1-related two-sided plane group. The crystallographic unit cell contains four monomeric CymA molecules showing a central pore. The lattice parameters are a = 16.1 nm, b = 3.8 nm,  = 93°. CymA does not form trimeric complexes in lipid membranes and shows no tendency to trimerize in solution. CymA thus is an atypical porin with novel properties specialized to transfer cyclodextrins across the outer membrane.

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