Two proteins that control pigmentation and body weight in animals, agouti (pronounced "ah-GOO-tee") protein and agouti-related protein, respectively, diverged from a common ancestor early in vertebrate evolution. The UCSC lab of Glenn Millhauser, professor of chemistry and biochemistry, has elucidated the structural differences that account for different functions for the two proteins.
These findings appeared as the featured article in the December 2006 issue of Chemistry & Biology.
The agouti protein (ASIP) is produced in the skin and binds to a receptor that controls fur patterns and shadings in some animals, such as the tabby pattern in cats, where each hair has several bands of color.
The agouti-related protein (AgRP) has a similar sequence and structure but a totally different function. AgRP resides in the brain, where it promotes increased feeding and decreased energy expenditure. Understanding this protein offers the possibility of controlling its function to treat diabetes, obesity, and other metabolism-related disorders.
The Millhauser lab determined the chemical structures of the two related proteins. Both have two important functional components, a C-terminal region and an N-terminal region. While the C-terminal regions are quite similar between the two agouti proteins, structurally and biochemically, the N-terminal regions differ significantly.
Work on the agouti protein that controls pigmentation has shown that the N-terminal region plays a critical role in the protein's function. Millhauser explained, "Based on this, it's been thought that the N-terminal in AgRP has a parallel role and must be critical for AgRP function. In fact, a famous 2001 Cell paper provided evidence for this."
But the new research shows just the opposite, Millhauser said. "The N-terminal domain of AgRP has to be cleaved away for full, mature AgRP function."
While the N-terminal region of the agouti protein enhances function, the corresponding domain in AgRP actually decreases function. "Our paper overturns a significant paradigm in AgRP function," Millhauser added.
The research in the Millhauser lab was conducted by graduate student Pilgrim Jackson, now a postdoc at UC Irvine, and undergraduate Nick Douglas, now in graduate school at Stanford. They collaborated on this project with two groups at Stanford University: the lab of Gregory Barsh, who discovered the agouti-related protein, and the Arend Sidow lab, which conducted the bioinformatic research on the results.
The bioinformatic analysis of the genetic origins of the two proteins show that agouti and its counterpart AgRP both come from a common ancestor. To reach this result, the Sidow lab employed evolution structure function (ESF) analysis.
Using ESF, they measured and compared local evolutionary rates for both proteins to reveal those regions that appear to have been constrained through the process of evolution. Because they have not been allowed to change by evolution, such regions tend to be the most important to maintaining a protein's structure and function.
The N-terminal regions of the two proteins show distinctly different patterns of evolutionary constraint, which relates to a divergence in their functions as well. This would have occurred after a duplication of the original gene somewhere in the genome, allowing the original and its copy to evolve in different directions.
"The ESF analysis provides a really nice example of how paralogsâ€”similar genes in different parts of the genome, which are often active in different tissuesâ€”can evolve to opposite functions," Millhauser explained.
The significance of these findings are three-fold, according to Millhauser: "We identified a new function for the N-terminal domain of AgRP, we explained a fundamental difference between agouti and AgRP, and we have established an interesting basis for agoutiâ€“AgRP evolution."