Let there be a channel

Anewly discovered rhodopsin does double duty as light detector and proton channel, say Georg Nagel (Max-Planck-Institut für Biophysik, Frankfurt am Main, Germany), Peter Hegemann (Universität Regensburg, Germany), and colleagues. Meanwhile, a group led by John Spudich at the University of Texas (Houston, TX) has used RNA interference to show that this and another channel are the two rhodopsins that the alga Chlamydomonas reinhardtii uses to determine whether to move closer to or further from a light source.

Previously, Hegemann has purified algal proteins that bind retinal, the light-detecting chromophore, but they turned out not to be the detector for phototaxis. His latest candidate, which was named channelrhodopsin-1 (Chop 1), came from a database search. The sequence is similar to that of bacteriorhodopsin, which passes protons from one residue to another thus acting as a pump. Although this network of residues is intact in Chop 1, some of these Chop 1 residues can be mutated without destroying proton conductance. Furthermore, Chop 1 produced in frog oocytes cannot move protons against a gradient, as with the pump bacteriorhodopsin, but acts like a channel that allows diffusion in either direction.The researchers are not surprised by the two-in-one protein, as the speed with which Chlamydomonas reacts to light had suggested a direct connection between light detection and ion conductance. Now, they want to understand how a pump like bacteriorhodopsin can be modified to make a channel that may be gated by retinal. ▪

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