Structure-Function Relationship of a Plant NCS1 Member – Homology Modeling and Mutagenesis Identified Residues Critical for Substrate Specificity of PLUTO, a Nucleobase Transporter from Arabidopsis

• Published in: PloS one Vol. 9; no. 3; p. e91343
• Main Authors: Witz, Sandra, Panwar, Pankaj, Schober, Markus, Deppe, Johannes, Pasha, Farhan Ahmad, Lemieux, M. Joanne, Möhlmann, Torsten
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 12.03.2014; Public Library of Science (PLoS)
• Subjects: Adenine; Amino Acid Sequence; Amino acids; Analysis; Arabidopsis; Arabidopsis Proteins - chemistry; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; Arabidopsis thaliana; Aspergillus nidulans; Binding sites; Binding, Competitive; Biochemistry; Bioinformatics; Biology; Calcium-binding protein; Catalysis; Chain dynamics; Competition; Computer simulation; Crystal structure; Docking; E coli; Escherichia coli; Family relations; Glycine; Guanine; Homology; Hydantoin; Lactose; Membrane permeability; Metabolism; Molecular Docking Simulation; Molecular dynamics; Molecular Sequence Data; Mutagenesis; Mutagenesis, Site-Directed; Mutants; Mutation; Nitrogen; Nucleobase Transport Proteins - chemistry; Nucleobase Transport Proteins - genetics; Nucleobase Transport Proteins - metabolism; Permeability; Physiology; Plant growth; Plastids; Proteins; Purines; Pyrimidines; Residues; Saccharomyces cerevisiae; Salvage; Sequence Homology, Amino Acid; Simulation; Structure-Activity Relationship; Structure-function relationships; Substrate Specificity; Substrates; Transport; Uracil
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0091343

Plastidic uracil salvage is essential for plant growth and development. So far, PLUTO, the plastidic nucleobase transporter from Arabidopsis thaliana is the only known uracil importer at the inner plastidic membrane which represents the permeability barrier of this organelle. We present the first homology model of PLUTO, the sole plant NCS1 member from Arabidopsis based on the crystal structure of the benzyl hydantoin transporter MHP1 from Microbacterium liquefaciens and validated by molecular dynamics simulations. Polar side chains of residues Glu-227 and backbones of Val-145, Gly-147 and Thr-425 are proposed to form the binding site for the three PLUTO substrates uracil, adenine and guanine. Mutational analysis and competition studies identified Glu-227 as an important residue for uracil and to a lesser extent for guanine transport. A differential response in substrate transport was apparent with PLUTO double mutants E227Q G147Q and E227Q T425A, both of which most strongly affected adenine transport, and in V145A G147Q, which markedly affected guanine transport. These differences could be explained by docking studies, showing that uracil and guanine exhibit a similar binding mode whereas adenine binds deep into the catalytic pocket of PLUTO. Furthermore, competition studies confirmed these results. The present study defines the molecular determinants for PLUTO substrate binding and demonstrates key differences in structure-function relations between PLUTO and other NCS1 family members.