ESTIMATION OF THE PORE-SIZE OF THE LARGE-CONDUCTANCE MECHANOSENSITIVEION-CHANNEL OF ESCHERICHIA-COLI

The open channel diameter of Escherichia coli recombinant large-conduc tance mechanosensitive ion channels (MscL) was estimated using the mod el of Hille (Hille, B. 1968. Pharmacological modifications of the sodi um channels of frog nerve. J. Gen. Physiol. 51:199-219)that relates th e pore size to conductance. Based on the MscL conductance of 3.8 nS, a nd assumed pore lengths, a channel diameter of 34 to 46 Angstrom was c alculated. To estimate the pore size experimentally, the effect of lar ge organic ions on the conductance of MscL was examined. Poly-L-lysine s (PLLs) with a diameter of 37 Angstrom or larger significantly reduce d channel conductance, whereas spermine (similar to 15 Angstrom), PLL1 9 (similar to 25 Angstrom) and 4,4'-bipyridinium)-1-yl)-propyl)-4,4'-b ipyridinium (similar to 30 Angstrom) had no effect. The smaller organi c ions putrescine, cadaverine, spermine, and succinate all permeated t he channel. We conclude that the open pore diameter of the MscL is sim ilar to 40 Angstrom, indicating that the MscL has one of the largest c hannel pores yet described. This channel diameter is consistent with t he proposed homohexameric model of the MscL.