Orientation of the tryptophans responsible for the photoinactivation of nerve sodium channels

Abstract

UV irradiation of squid giant axons at wavelengths of 280 or 290 nm produces nearly the same rate of irreversible decrease of sodium currents. The rate of photodeactivation is unaffected by extensive removal of axoplasm with pronase, and it is independent of temperature in the range 5 degrees to 20 degrees C. The photochemical effect appears to be all or nothing. It does not alter the time course and the voltage dependence for activation and inactivation of the residual currents. Similar deactivation rates were produced by irradiations of the same intensity, but linearly polarized either parallel or perpendicular to the axon. The efficiency of the deactivation process is close to that expected if it was caused by the photooxidation of a single tryptophan residue per sodium channel. Owing to the geometry of the preparation the lack of polarization asymmetry suggests that this residue assumes nearly random (or pseudo-random) orientation in the three-dimensional structure of the sodium channel corresponding to the closed state.

Probing folding and fluorescence quenching in human γD crystallin Greek key domains using triple tryptophan mutant proteins

“…The existence of native‐state quenching of tryptophans in a protein exposed to ultraviolet light, but selected for its stability and solubility, may reflect a role in protecting it from the absorption events. Prolonged exposure to ultraviolet light or other oxidative conditions populated in lens over time by HγD‐Crys may generate ring opening and other covalent damage well characterized in other systems (Conti et al 1988; Balasubramanian et al 1990; Prinsze et al 1990). The generation of a charged species within the buried core of the crystallins would be expected to destabilize the protein and cause full or partial unfolding.…”

Implication of segment S45 in the permeation pathway of voltage-dependent sodium channels

“…$4 (domain II) also presents next to this Pro the only Trp of all the $4-$45 fragments. Since each SS1 -SS2 segment is endowed with 1 or 2 Trp, a photodeactivation study with those model peptides in planar lipid bilayers should discriminate which Trp is most responsible for the effect reported on sodium channels in squid giant axons (Conti et al 1988). No doubt, the strategy using model peptides fostered by the latest developments in automatic peptide synthesis will be further extended and ever more complement mutagenesis and cDNA expression investigations of structure-function relationships in ionic channels.…”