Electrostatic Tuning of Cftr's Open States | Zhang and Hwang 19 Jgp

Abstract

For this two-state model, the binding constant (kon/koff) is the sole determinant deciding the degree of block at a fixed concentration of MOPS−. Therefore, for the mutants with a net charge of 0 (unaltered) or 1, as long as this ratio (kon/koff) is larger for the O1 state than for the O2 state, the O1 state will be more sensitive to MOPS− block. One can propose that an expansion of the internal vestibule during the O1 to O2 transition decreases the electrostatic attraction of the pore for MOPS−, hence resulting in less blockade for the O2 state. However, for the mutants with a net charge of −1, volume expansion during the O1 to O2 transition will decrease the electrostatic “repulsion,” resulting in a larger (kon/koff) for the O2 state compared with that of the O1 state. At first glance, this prediction contradicts our experimental observation that O1 and O2 exhibit equal sensitivity to MOPS− for these mutants. But if we consider that for mutants with a net charge of −1, the binding constants (kon/koff) for both the O1 and O2 states are much smaller than those of WT so that the difference in Fub between the O1 and O2 states is too small to be resolved given a dramatic reduction of the single-channel amplitudes of both states (Figs. 2 and 8 D), we can still find proper parameters to fit our data with Scheme S1. However, this model lacks the physical details describing the assigned parameters and is oversimplified for not considering the direct contact between MOPS− and the wall of the pore. Furthermore, Scheme S1 also implies that the blocking site for MOPS− is in the internal vestibule. However, the generally held view of a wide space in the internal vestibule argues against a high voltage dependence of the block observed experimentally. After some reckoning, we decided to add one nonconductive docked state for MOPS− block (Scheme S2 and Fig. S4). To better explain our data with MOPS−, a “docking” step was introduced into Scheme S1 to derive Scheme S2:

Cite this paper

@inproceedings{Ta2017ElectrostaticTO, title={Electrostatic Tuning of Cftr's Open States | Zhang and Hwang 19 Jgp}, author={Shiying Ta and L M At and Annie L. Zhang and C B Hwang}, year={2017} }