protein can be responsive to green or yellow light and can be derived from halorubrum sodomense wherein the proton pump protein can be capable of mediating a hyperpolarizing current in the cell when the cell is illuminated with green or yellow light the light can have a wavelength between about 560 and about 570 nm or can have a wavelength of about 566 nm in another embodiment the light responsive proton pump protein can comprise an amino acid sequence at least about 90 91 92 93 94 95 96 97 98 99 or 100 identical to the sequence shown in seq id no 12 the light responsive proton pump protein can additionally comprise substitutions deletions and or insertions introduced into a native amino acid sequence to increase or decrease sensitivity to light increase or decrease sensitivity to particular wavelengths of light and or increase or decrease the ability of the light responsive proton pump protein to regulate the polarization state of the plasma membrane of the cell additionally the light responsive proton pump protein can contain one or more conservative amino acid substitutions and or one or more non conservative amino acid substitutions the light responsive proton pump protein comprising substitutions deletions and or insertions introduced into the native amino acid sequence suitably retains the ability to hyperpolarize the plasma membrane of a neuronal cell in response.