A proton switch in GFP

Emission from green fluorescent protein (GFP) is one of the most widely used methods of molecular marking in cell biology, since GFP can be prepared as a fusion protein with just about any other gene product. But the fluorescence behaviour is curious and hasn’t been fully explained. In particular, it shows a t**-3/2 time dependence in the long-time tail at room temperature, but switches to a t**-1/2 dependence below 230 K. Fluorescence involves a photoexcited proton transfer from the chromophore, which is thought to occur along a hydrogen-bonded chain involving various residues and bound water molecules. Noam Agmon has modelled this process, considering the ‘proton wire’ to be rather longer than is normally thought and to have within it a switch at a threonine residue (Thr203) with a large activation energy for proton migration (J. Phys. Chem. B 111, 7870; 2007). The rapid migration of the proton is held up at this switch point for typically 300 ps at room temperature. This model can explain both the t**-3/2 behaviour at room temperature and the changeover to different asymptotics at 230 K. Here’s another example of water and hydrogen-bonding residues collaborating to engineer biological function.