Scientists from The Scripps Research Institute have solved a 10-year-old mystery of how a single protein from an ancient family of enzymes can have two completely distinct roles in the body. The team, led by research associate Xiang-Lei Yang, Ph.D., focused on a molecule called human tryptophanyl-tRNA synthetase (TrpRS).
The findings were published December 13 in the advance, online issue of Nature Structural and Molecular Biology in an article titled “Orthogonal use of a human tRNA synthetase active site to achieve multifunctionality.”
Scientists have known for a while that the aminoacyl tRNA synthetase family is composed of 20 enzymes that attach the correct amino acid to a tRNA as the first step in the synthesis of proteins. Then in 1999, a group led by Paul Schimmel, Ph.D., member of The Skaggs Institute for Chemical Biology at Scripps Research, uncovered the protein family's dual functionality.
They reported in Science that a fragment of tyrosyl-tRNA synthetase (TyrRS) attracted immune cells to stimulate the growth of blood vessels besides simply adding the tyrosine amino acid to a protein chain during synthesis. Soon afterward, the Schimmel lab showed that another member of the family, TrpRS, also had a dual function. In addition to adding tryptophan to a protein chain, a fragment of TrpRS could inhibit new blood-vessel formation.
Until now, however, no one has been able to figure out exactly how they perform their different roles. In the current study, the Scripps group used a combination of techniques including structural modeling analysis, mutagenesis, and cell-based functional studies. They were able to determine the specific molecular changes that enabled TrpRS to perform one function or another.
The scientists found that for its role in protein synthesis, TrpRS is typically in its full-length form. This form of the molecule contains a tryptophan-binding pocket that enables it to bind with the amino acid and shepherd it to where it is needed in protein synthesis.
In the second active form, however, the protein is first broken into fragments by the body, creating a piece called T2-TrpRS. With the removal of the end of the full-length protein (the N-domain), new grooves in the T2-TrpRS protein fragment appear. Containing the now-exposed tryptophan-binding pocket, the grooves fit together with side chains of another molecule called VE-cadherin, which is known to be indispensable for proper vascular development.
The new study also found that tryptophan acts to inhibit vasculature function of TrpRS, locking the protein into its protein-synthesis form. Dr. Yang notes that the therapeutic potential of TrpRS and other tRNA synthetases are particularly good because they normally exist in abundant amounts in the body. “Naturally, you'd imagine the body's tolerance for such a protein is pretty good, and we could use the activated form of the molecule.” Dr. Yang also points out that TrpRS is intriguing because it does not affect existing blood vessel growth, only new blood vessel formation, reducing the odds of negative side effects from its use.