Arabidopsis thaliana 'extra-large GTP-binding protein' (AtXLG1): a new class of G-protein.
Heterotrimeric GTP-binding proteins, composed of alpha, beta, and gamma subunits, are involved in signal transduction pathways in animal and plant systems. In plants, physiological analyses implicate heterotrimeric G-proteins in ion channel regulation, light signaling, and hormone and pathogen responses. However, only one class of plant G alpha genes has been identified to date. We have cloned a novel gene, 'Arabidopsis thaliana extra-large GTP-binding protein' (AtXLG1). AtXLG1 appears to be a member of a small gene family and is transcribed in all tissues assayed: roots, leaves, stems, flowers, and fruits. The conceptually translated protein from AtXLG1 is 99 kDa, twice as large as typical G alpha proteins. The carboxy-terminal half of the AtXLG1 protein has significant homology to animal and plant G alpha proteins. This region includes a GTP-binding domain, a predicted helical domain, and an aspartate/glutamate-rich loop, which are characteristics of G alpha's. Despite the absence of some of the amino acids implicated in GTP binding and hydrolysis by crystallographic and mutational analyses of mammalian G alpha's, recombinant AtXLG1 binds GTP with specificity. The amino-terminal region of AtXLG1 contains domains homologous to the bacterial TonB-box, which is involved in energy transduction between the inner and outer bacterial membranes, and to zinc-finger proteins. Given the unique structure of AtXLG1, it will be of interest to uncover its physiological functions.
Department of Biology, Pennsylvania State University, University Park 16802, USA.
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Lee
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Assmann
D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics*; D029681:Arabidopsis Proteins; D001426:Bacterial Proteins; D017124:Conserved Sequence; D003545:Cysteine; D018076:DNA, Complementary / Q000235:genetics; D019143:Evolution, Molecular; D019204:GTP-Binding Proteins / Q000145:classification* / Q000235:genetics*; D017343:Genes, Plant; D015698:Genomic Library; D006160:Guanosine Triphosphate / Q000378:metabolism*; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000145:classification / Q000235:genetics; D017386:Sequence Homology, Amino Acid
D029681:Arabidopsis Proteins; D001426:Bacterial Proteins; D018076:DNA, Complementary; D008565:Membrane Proteins; D010940:Plant Proteins; C120055:XLG1 protein, Arabidopsis; C057616:tonB protein, Bacteria; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D003545:Cysteine