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There is now a CONTENT FREEZE for Mercury while we switch to a new platform. It began on Friday, March 10 at 6pm and will end on Wednesday, March 15 at noon. No new content can be created during this time, but all material in the system as of the beginning of the freeze will be migrated to the new platform, including users and groups. Functionally the new site is identical to the old one. webteam@gatech.edu
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Prof. Noam Agmon, Hebrew University Jerusalem
Proton wires and proton exit in the green fluorescent protein
The Green Fluorescence Protein (GFP) from the jellyfish Aequorea Victoria has attracted much interest as a biological fluorescence marker and an example of excited-state proton transfer (ESPT) in nature. The proton was assumed to migrate irreversibly in the ES to Glu222. In collaboration with Dan Huppert (Tel-Aviv), we have studied the transient fluorescence of the wild-type (wt) protein over a wide range of time and temperature. Unexpectedly, the decay of the acid form of wt-GFP is non-exponential, exhibiting two different power-laws. Below ca. 230 K, we observe a t-1/2 power-law decay, suggesting one-dimensional diffusion along a pre-formed "proton-wire" within the protein. Indeed, the x-ray structure reveals such a proton-wire, extending beyond Glu-222, on which the proton is apparently trapped at low T. If this interpretation is correct, it provides a first demonstration of proton-wire dynamics in real time.
Above 230 K, the time-resolved kinetics drop below the t-1/2 asymptotics, exhibiting a t-3/2 behavior near room-temperature. We show that this phase is not due to three-dimensional diffusion as previously postulated. It stems from an added irreversible sink term to the one-dimensional diffusion equation, depicting irreversible proton exit to solution. The x-ray data reveals that a "threonine switch" (Thr-203) apparently opens above 230 K, establishing a short exit route. The interpretation is corroborated by analysis of the kinetics of the S205V mutant, in which the proton-wire is eliminated, but not the threonine switch. Accordingly, we see in this case slow, exponential proton exit controlled by the conformational change, but no power-laws attributable to diffusion along the proton-wire.
For more information contact Dr. Laren Tolbert (404-894-4093) or Dr. Kyril Solntsev (404-385-4078).
