Vertebrate photoreceptor cells can signal the absorption of a single p
hoton and then modulate their response as the intensity of the light a
nd the intensity of the background illumination vary, and it has long
been recognized that Ca2+ ions contribute to the underlying processes.
Recently, several Ca2+-binding proteins of the EF-hand family were id
entified that mediate the actions of Ca2+ during the response to light
, Molecular interactions between these Ca2+-binding proteins and their
cellular targets are amenable to study owing in part to the unique fe
atures of phototransduction. In addition,two of the proteins, recoveri
n and guanylate cyclase activating protein (GCAP), appear to be involv
ed in separate degenerative diseases of the retina that arise in human
s and in animal models of human disease, Information obtained from the
se studies should also be relevant to the growing number of homologous
proteins found in other neural tissues.