STRUCTURE AND FUNCTIONAL MECHANISM OF PORINS.
JAP, BING K. AND PETER J. WALIAN.
Donner Laboratory, Life Sciences Division, Lawrence Berkeley National
Laboratory, University of California, Berkeley, California
APStracts 2:0036P, 1996.
ABSTRACT
There is substantial evidence of voltage-dependent gating of porin channels
under in vitro conditions. Planar lipid bilayer and patch-clamp experiments
show porin voltage closure as a cascade of three unitary steps in response to
an applied potential difference; in some cases, a potential difference of as
high as 200 mV has been used. This closure pattern is consistent with the
notion that trimeric porin contains three independent pores (see, for example,
Refs. 6, 11, 40, 44, 66, 84). It is interesting to note here that the
maltodextrin-specific porin LamB, which also contains three independent pores,
exhibits a probability of channel gating that is voltage independent (12). As
described earlier,the distribution of ionizable residues at the constriction
zone in this channel is quite different from that seen in the nonspecific
bacterial porins and may play a role in channel sensitivity to transmembrane
potential differences. For some nonspecific porins, a residual conductance has
been measured for channels in the closed state (11, 44, 79; H. Lecar, personal
communication). A nonzero closed state conductance is also observed for other
porinlike channels such as VDAC (3) and "excitability-inducing material" (EIM)
(19). Closed porin channel reopenings have been reported (5, 6, 8, 11, 14, 40,
44, 66, 84); reopenings in the form of three sequential unitary step
conductance increases can also be observed (Lecar, personal communication).
The closed-state dwell time for the porin channels as measured on groups of
reconstituted channels as well as single-channel observations (see, for
example, Refs. 6, 40, 44, 77; Lecar, personal communication) varies with
experimental conditions and can be up to several minutes long. This is in
clear contrast to other voltage-regulated channels such as the K[sup]+[r]
[i]Shaker[r] channel which has millisecond dwell times (29) and EIM channels
which has second dwell times (19).
APS Manuscript Number P12-6.
Article publication pending October 1996, Physiological Reviews.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 13 November 1996