Molecular Mechanism of Visual Transduction

Our vision renders an incredible wealth of information about the external environment presented in the form of light of different wavelengths and intensities. To operate in a wide range of light intensities, our visual system has developed several mechanisms that allow an adjustment of its sensitivity to light. Immense progress has been made in understanding how light is captured and activates visual phototransduction cascade within photoreceptor cells; however, much less is known about desensitization. It has been known for some time, that many of these processes rely on Ca2+ as the principal modifier of phototransduction. Ca2+-binding proteins (CBPs) are specifically poised to take advantage of transient changes in [Ca2+] to act as enzymatic regulators. Some other CBPs are capable of changing the intracellular Ca2+ buffering capacity. Various retinal CBP proteins have been identified, including recoverin, GCAP1, GCAP2, GCAP3, GCIP, CBP1, CBPS and CBP4. Although these numerous CBPs were identified, functions can be ascribed to only a few of them. Recently, genetic, physiological and biochemical analyses of retinal diseases have yielded additional insights into the role of many phototransduction proteins, including CBPs. Understanding the properties and the functions of these CBPs will pave the way for a more complete picture of visual transduction and accompanying desensitization processes.