Abstract

The rod photoreceptor of the retina is a quantum detector whose physiological function in physically unfavorable conditions (body temperature, salt solution) is made possible by specific protein interactions. Photon energy is stored by the receptor protein rhodopsin (R) in a structurally transformed state. Activated R interacts with transducin (a G-protein or guanine nucleotide binding protein). This catalyses binding to G of energy-rich nucleotide which in turn releases G in an activated form. Absorption of one photon leads to the activation of 1000 G in 1 s. Analogous relay systems are found from bacteria to man. Intrinsic physical properties of the rhodopsin G-protein system allow photometric studies in situ and in real time. Activation of R and interaction with G are measurable by absorption spectrophotometry. Activation of G is measurable by light scattering (LS) changes (signals) arising from the shift of the G-protein mass during activation. A continuous transretinal, near infrared LS probing beam affords direct monitoring of G-activation induced by visual stimuli. These optical techniques, combined with biochemical and physiological approaches, have been used to study the sites of R-G interaction and the thermodynamics of the G-relay in situ. G-activation is not modulated by previous illumination, indicating a remarkable constancy of the R-G amplification step in the visual transduction pathway.

© 1989 Optical Society of America