Deep within the brains of frogs, in a part of their anatomy where the sun never shines, appears to be a protein that catalyzes biochemical reactions in response to light. The curious finding, reported in the current issue of the Proceedings of the National Academy of Sciences, could help researchers better understand how light sets the biological clocks that tell frogs--and perhaps humans--when to perform a host of basic functions, including sleep and metabolism.
The protein, called melanopsin, first came to light during studies of melatonin, a hormone associated with human sleep cycles, says Ignacio Provencio, a neurobiologist at the Uniformed Services University of Health Sciences in Bethesda, Maryland. When analyzing proteins from pigmented skin cells of the African clawed frog, Provencio's team found messenger RNA that helps make a new opsinlike protein--although they have not isolated the protein itself. Opsins change shape in response to light and initiate a chain of chemical reactions, which are eventually converted into nerve impulses.
Expanding their search to other frog tissues, the researchers found more evidence of the protein in the nonoptical cells of the retina, in the iris, and deep in the brain. That makes sense, says Provencio, because melanopsin appears to be a large protein, indicating that it is probably complex enough to have functions in many types of tissue. The protein's presence in skin and in the brain suggests a connection to circadian rhythms, an organism's response to cycles of light and darkness, that are controlled by the brain.
If melanopsin turns out to be light-sensitive, experts say, it could lead researchers further into the shadow world of light-converting proteins. "We're still at the level where we don't know all the photosensitive pigments, how they work, or how they relate to circadian rhythms," says Marianna Max, a neuroscientist at Mount Sinai Medical Center in New York City. Melanopsin should "give us a simpler model." However, adds Provencio, it's questionable how much melanopsin can tell us about other species: The protein shares only 39% of the genetic code of its closest known relative, an octopus opsin.