Ca2+-binding proteins in the retina: structure, function, and the etiologyof human visual diseases

The complex sensation of vision begins with the relatively simple photoisom erization of the visual pigment chromophore 11-cis-retinal to its all-trans configuration. This event initiates a series of biochemical reactions that are collectively referred to as phototransduction, which ultimately lead t o a change in the electrochemical signaling of the photoreceptor cell, To o perate in a wide range of light intensities, however, the phototransduction pathway must allow for adjustments to background light. These take place t hrough physiological adaptation processes that rely primarily on Ca2+ ions. While Ca2+ may modulate some activities directly, it is more often the cas e that Ca2+-binding proteins mediate between transient changes in the conce ntration of Ca2+ and the adaptation processes that are associated with phot otransduction. Recently, combined genetic, physiological, and biochemical a nalyses have yielded new insights about the properties and functions of man y phototransduction-specific components, including some novel Ca2+-binding proteins. Understanding these Ca2+-binding proteins will provide a more com plete picture of visual transduction, including the mechanisms associated w ith adaptation, and of related degenerative diseases. BioEssays 22:337-350, 2000. (C) 2000 John Wiley & Sons, Inc.