One of the best ways to track the voltage of an ion channel is to insert a green flouroscent protein. When the voltage in the ion channel changes, the wavelength emitted (color, in nanometers) will also change, making voltage tracking much easier.
With potassium channels, this has been more challenging. The natural interaction of nearby subunits may be so great that if one section is truncated and replaced with a GFP protein, the protein will not correctly fold and insert into the plasma membrane.
In order to overcome this difficulty, researchers Dimitrov et al. identified a voltage-sensing region in Ci-VSP, a non-ion channel in Ciona instestinalis (for Ci). They speculated that this channel would be less dependent on the interactions with other proteins and thus fit into the plasma membrane well.
After fusing the fluorescent proteins (FP) to Ci-VSP, the researchers tested their efficiency in PC12 cells, often used in neuronal differentiation because they terminally differentiate when treated with nerve growth factor. The fusion products yielded clear fluorescence and targeting to the plasma membrane of these PC12 cells.
Their technique involved transfecting the FP – CI-VSP fusion into PC12 cells and analyzing their voltage using a voltage clamp. The protein emitted more yellow (>515 nanometers) and less less cyan (460-500 nm) during an increase in voltage. This was the effect that they were looking for, as it means that the changes in the relative amounts of these colors could be used as a way to measure the voltage of a cell.
Because of the protein’s improved ability to target the plasma membrane and reasonably large voltage dependent signal, this new technique represents an improvement flourescent protein voltage sensors in monitoring neuronal activity. If coupled with other measures, it could be used to determine voltage dependence on external and internal properties of a cell.
Dimitrov D, He Y, Mutoh H, Baker BJ, Cohen L, et al. 2007 Engineering and Characterization of an Enhanced Fluorescent Protein Voltage Sensor. PLoS ONE 2(5): e440 doi:10.1371/journal.pone.0000440