Voltage-dependent closing of porin channels: analysis of relaxation kinetics

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.