Nz. Liu et al., DISRUPTION OF POLYAMINE MODULATION BY A SINGLE AMINO-ACID SUBSTITUTION ON THE L3 LOOP OF THE OMPC PORIN CHANNEL, Biochimica et biophysica acta. Biomembranes, 1326(2), 1997, pp. 201-212
Structural studies have demonstrated that the extracellular L3 loop of
porin constricts the channel and suggest that this loop might be invo
lved in channel selectivity and gating. We previously showed that posi
tively charged polyamines can induce changes in porin gating kinetics
by stabilization of closed states. Here we report the effects of the m
utation of two different aspartate residues of Escherichia coli OmpC p
orin on the polyamine sensitivity of the channel. Aspartate 105 or asp
artate 118 on the L3 loop was replaced by glutamine by site-directed m
utagenesis. The gating activity of the wild-type and mutant channels w
ere studied by patch-clamp of liposomes containing reconstituted outer
membrane fractions, in the absence or the presence of either polyamin
e spermine or cadaverine. Porin channels with a D118Q mutation, at the
root of L3, still showed some, albeit milder, sensitivity to polyamin
e modulation. On the other hand, the D105Q mutation, at the tip of L3,
abolished the increase in closing frequency which is typically observ
ed in the presence of polyamines. We conclude that aspartate 105 prima
rily, but not aspartate 118, plays an important role in mediating the
polyamine-induced changes in gating kinetics that result in the inhibi
tion of the OmpC channel.