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Leading Edge 246 Cell 139, October 16, 2009 2009 Elsevier Inc.

Summary: Leading Edge
246 Cell 139, October 16, 2009 2009 Elsevier Inc.
The great majority of animals have photoreceptors of one sort or
another for detecting food source, mate, predator/prey, orienta-
tion, or simply the light/dark cycle dictated by the movement of
the sun. Such photoreceptors, whether ocular or extraocular, are
generally distinguishable into two types: ciliary and rhabdomeric,
depending on whether the proliferation of photosensitive mem-
branes necessary for efficient light absorption is derived from a
modified cilium or from microvillar projections of the apical cell
surface forming a rhabdom (for review, Arendt, 2003; Lamb et al.,
2007). Uniformly, these photoreceptors sense light with a visual
pigment composed of a vitamin A-based chromophore and a
seven-transmembrane-helix apoprotein, opsin. These pigments
are prototypical G protein-coupled receptors (GPCRs), signaling
via heterotrimeric G proteins. More than 1000 opsins have now
been identified in the animal kingdom, all believed to originate
from a common ancestor and separate from the structurally simi-


Source: Alford, Simon - Department of Biological Sciences, University of Illinois at Chicago


Collections: Biology and Medicine