2q0c
Terminal uridylyl transferase 4 from Trypanosoma brucei with bound CTPTerminal uridylyl transferase 4 from Trypanosoma brucei with bound CTP
Structural highlights
FunctionTUT4_TRYB2 Terminal uridylyltransferase which, specifically, catalyzes the addition of Us to the 3'-hydroxyl group of single-stranded RNAs with a 3'-terminal U.[1] [2] Evolutionary Conservation Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf. Publication Abstract from PubMedTerminal RNA uridylyltransferases (TUTases) catalyze template-independent UMP addition to the 3' hydroxyl of RNA. TUTases belong to the DNA polymerase beta superfamily of nucleotidyltransferases that share a conserved catalytic domain bearing three metal-binding carboxylate residues. We have previously determined crystal structures of the UTP-bound and apo forms of the minimal trypanosomal TUTase, TbTUT4, which is composed solely of the N-terminal catalytic and C-terminal base-recognition domains. Here we report crystal structures of TbTUT4 with bound CTP, GTP, and ATP, demonstrating nearly perfect superposition of the triphosphate moieties with that of the UTP substrate. Consequently, at physiological nucleoside 5'-triphosphate concentrations, the protein-uracil base interactions alone are not sufficient to confer UTP selectivity. To resolve this ambiguity, we determined the crystal structure of a prereaction ternary complex composed of UTP, TbTUT4, and UMP, which mimics an RNA substrate, and the postreaction complex of TbTUT4 with UpU dinucleotide. The UMP pyrimidine ring stacks against the uracil base of the bound UTP, which on its other face also stacks with an essential tyrosine. In contrast, the different orientation of the purine bases observed in cocrystals with ATP and GTP prevents this triple stacking, precluding productive binding of the RNA. The 3' hydroxyl of the bound UMP is poised for in-line nucleophilic attack while contributing to the formation of a binding site for a second catalytic metal ion. We propose a dual role for RNA substrates in TUTase-catalyzed reactions: contribution to selective incorporation of the cognate nucleoside and shaping of the catalytic metal binding site. Dual role of the RNA substrate in selectivity and catalysis by terminal uridylyl transferases.,Stagno J, Aphasizheva I, Aphasizhev R, Luecke H Proc Natl Acad Sci U S A. 2007 Sep 11;104(37):14634-9. Epub 2007 Sep 4. PMID:17785418[3] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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