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 peptidecontaining 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 M 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.

The outer membranes of the 10 serovars of Vibrio anguillarum showed a common major protein with a size of around 40 kDa. Antibodies against the major outer membrane protein (MOMP) of V. anguillarum AO18 (serovar O1) cross-reacted with the MOMPs of all the other serovars but not with the outer membrane proteins of Escherichia coli. The MOMP of V. anguillarum serovar O1 was isolated, reconstituted to two-dimensional crystals, and structurally characterized by electron microscopy and image processing. The unit cell structure of the crystalline MOMP, as well as the secondary structure composition of the protein with a high amount of ␤-structure, is strongly reminiscent of that of bacterial porins. The functional properties of the pores were investigated by conductance measurements with the MOMP reconstituted in planar lipid membranes. The V. anguillarum MOMP is characterized by a relatively weak cation selectivity and a moderate surface charge, and it shows voltage-dependent conductance effects. The MOMP is functionally similar to OmpF from E. coli, and it can be classified as a general diffusion porin.Vibrio anguillarum, a gram-negative bacterium, is the etiological agent of vibriosis, one of the most serious infection diseases of salmonid and marine fish species throughout the world (2). The diagnosis and control of V. anguillarum infections are of great economical importance in commercial fish farming.Since the first descriptions of V. anguillarum infections, many studies on the immunological characterization of this species and its pathogenicity have been performed. The molecular basis of these studies is closely related to the composition and function of the outer membrane (OM). Most studies of the V. anguillarum OM have been focused on the lipopolysaccharides (LPS) which are the major agents for antigenetic specificity (11). Several serotyping systems based on the slide agglutination method with thermostable O-antigens were described independently (29, 47). These serovars are considered an important epizootical characteristic. The three main serovars O1, O2, and O3 (according to the classification of Sør-ensen and Larsen [47]) include most of the strains isolated from fish infections. The remaining serovars are more commonly isolated from the natural environment and other aquatic species (47). Nevertheless, there is also an increasing interest in the OM proteins of V. anguillarum which may participate in mechanisms of pathogenicity directly. So, special attention has been paid the OM proteins involved in siderophore-dependent iron uptake (13,34). Several studies analyzed and compared the compositions of OM proteins from different serovars (1, 40). They describe the presence of usually one major OM protein (MOMP) in the molecular size range of about 30 to 40 kDa in all the strains tested. Recently, Suzuki et al. (48) suggested that this MOMP represents a porin because of its molecular weight.In the present work, we have studied the OM protein profiles of different serovars of V. anguillarum. The MOMP of one str...

The open channel characteristics of the bacterial porin Omp32 from Comamonas acidovorans were investigated by means of conductance measurements in planar lipid bilayers of the Montal-Mueller type. Particularly at low salt conditions (< or = 30 mM KCl) Omp32 exhibited some unusual asymmetric and nonlinear functional properties. Current-voltage relationship measurements showed that conductance depends on the orientation of porin molecules and is a nonlinear function of the applied membrane potential. Conductance also depends on the salt concentration in a manner not common to porins and the salt concentration modulates the nonlinearity of conductance-voltage relationships. Omp32 is strongly anion-selective. The nonlinear and asymmetric conductance of the open channel is a new observation in porins.

The anion-selective porin Omp34 from Acidovorax delafieldii was unidirectionally reconstituted in planar lipid membranes. Pore closing was recorded particularly at low salt conditions for negative and positive membrane potentials in the range of +/-10 to +/-100 mV. Relaxation curves were fitted by exponential functions in order to describe and to analyze the voltage-dependent behavior. Omp34 exhibited the following characteristics: (i) The channels are asymmetric with respect to closing characteristics and corresponding functional parameters. (ii) Relaxation curves can be fitted by a single exponential function in the low voltage range only, at >/=40 mV combinations of two exponential functions are required. (iii) Beyond 60 to 70 mV a third exponential function is necessary to fit the fast closing components properly. The time constants differ by two to three orders of magnitude. (iv) Hysteresis in I-V-diagrams originate from slow relaxation components which are different for positive and negative voltages. The implications for models aiming at description of voltage-dependent closing are discussed.