The ion-coupled sugar membrane symporter or co-transporter melibiose permease (MelB), responsible for alpha-galactoside accumulation in Escherichia coli, is a representative member of the glycoside-pentoside- hexuronide family of the vast class of electrochemical potential-driven porters. Pure solubilized preparations of a MelB recombinant protein were subjected to two-dimensional crystallization trials and several crystal forms were observed. Two of these appeared as large wide tubes suitable for analysis by electron crystallography. Flattened tubes on carbon support film, embedded in amorphous ice prior to electron cryomicroscopy, showed two-sided plane group symmetries P12(1) or P222(1), with unit cell dimensions a = 89.9 A, b = 51.6 A, gamma = 91.9 degrees and a = 188.9 A, b = 48.8 A, gamma = 90 degrees, respectively. The projection map from the P222(1 )crystals at 8 A resolution displayed an asymmetric protein unit consisting of two domains lining a central and curve-shaped cleft. Together, the MelB monomer could host the 12 predicted transmembrane alpha-helices. Overall, the MelB helix packing arrangement compared more favorably with that of the Na(+)/H(+) antiporter NhaA than that of the oxalate antiporter.

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Two-dimensional crystallization of membrane-bound H,K-ATPase (EC 3.6.1.36) in vesicle prcpamtions from parictal cells of hog gastric mucosa was induced by an imidazole buffer containing Mg" and VO>-ions. A continuous reorgani7aiton of the protein molecules started within a few hours by the formation of linear arrays. At later stages confluent two-dimensional crystals were formed. Electron microscopy and image processing showed that these were of a single tctragonal rype, The asymmetric unit consisted of one pear-shaped protein domain corresponding to a H,K-ATPase protomer. Through stain-deficient contxt regions four adjacent protein units were connected fomling a tetrameric structure.

The semi-rigid amphetamine analogue phenmetrazine crystallizes as the hydrochloride in both the orthorhombic and the monoclinic systems. Monoclinic crystals with a= 6.11, b=29.58, c=7"19 A, fl= 112.4 °, space group Cc and four molecules of CllHIsON. HC1 in the unit cell were used for the investigation. The structure was determined from 1098 unique and significant counter-recorded reflexions within sin 0]2<0-65 by the heavy-atom procedure and direct methods independently and was refined to an R index of 0.055. The conformation of the molecule was found to be almost identical with corresponding parts of amphetamine. The structure contains two strong NH...C1 hydrogen bonds with N.• • CI distances of 3"09 and 3"07 A which give rise to infinite layers of phenmetrazine and chloride ions. The layers are held together by van der Waals forces.

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