title: Two crystal structures reveal design for repurposing the C-Ala domain of human AlaRS.
abstract: The 20 aminoacyl tRNA synthetases (aaRSs) couple each amino acid to their cognate tRNAs. During evolution, 19 aaRSs expanded by acquiring novel noncatalytic appended domains, which are absent from bacteria and many lower eukaryotes but confer extracellular and nuclear functions in higher organisms. AlaRS is the single exception, with an appended C-terminal domain (C-Ala) that is conserved from prokaryotes to humans but with a wide sequence divergence. In human cells, C-Ala is also a splice variant of AlaRS. Crystal structures of two forms of human C-Ala, and small-angle X-ray scattering of AlaRS, showed that the large sequence divergence of human C-Ala reshaped C-Ala in a way that changed the global architecture of AlaRS. This reshaping removes the role of C-Ala in prokaryotes for docking tRNA and instead repurposes it to form a dimer interface presenting a DNA-binding groove. This groove cannot form with the bacterial ortholog. Direct DNA binding by human C-Ala, but not by bacterial C-Ala, was demonstrated. Thus, instead of acquiring a novel appended domain like other human aaRSs, which engendered novel functions, a new AlaRS architecture was created by diversifying a preexisting appended domain.
authors.affiliation: The Scripps Laboratories for tRNA Synthetase Research, The Scripps Research Institute, La Jolla, CA 92037.|Department of Cell and Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037.
authors.forename: Litao
authors.initials: L
authors.lastname: Sun
authors.affiliation: The Scripps Laboratories for tRNA Synthetase Research, The Scripps Research Institute, La Jolla, CA 92037.|Department of Cell and Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037.
authors.forename: Youngzee
authors.initials: Y
authors.lastname: Song
authors.affiliation: The Scripps Laboratories for tRNA Synthetase Research, The Scripps Research Institute, La Jolla, CA 92037.|Department of Cell and Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037.
authors.forename: David
authors.initials: D
authors.lastname: Blocquel
authors.affiliation: The Scripps Laboratories for tRNA Synthetase Research, The Scripps Research Institute, La Jolla, CA 92037.|Department of Cell and Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037.
authors.forename: Xiang-Lei
authors.initials: XL
authors.lastname: Yang
authors.affiliation: The Scripps Laboratories for tRNA Synthetase Research, The Scripps Research Institute, La Jolla, CA 92037; schimmel@scripps.edu.|Department of Cell and Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037.|Department of Metabolism and Aging, The Scripps Florida Research Institute, Jupiter, FL 33458.
authors.forename: Paul
authors.initials: P
authors.lastname: Schimmel
mesh_terms: D000412:Alanine-tRNA Ligase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D019843:Archaeal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D019611:Archaeoglobus fulgidus / Q000201:enzymology / Q000235:genetics; D001665:Binding Sites; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D004247:DNA / Q000378:metabolism; D019143:Evolution, Molecular; D006801:Humans; D007700:Kinetics; D008958:Models, Molecular; D000072417:Protein Domains; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D017386:Sequence Homology, Amino Acid; D055672:Static Electricity
chemical_list: D019843:Archaeal Proteins; D004247:DNA; D000412:Alanine-tRNA Ligase
keywords: DNA binding; appended domain; evolution; splice variant; structural plasticity