2i5s

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spinqualitylabelsall models 2i5s, resolution 1.900Å Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Crystal structure of onconase with bound nucleic acid

Overview

Onconase (ONC) is a homolog of bovine pancreatic ribonuclease (RNase A), from the frog Rana pipiens. ONC displays antitumoral activity and is in, advanced clinical trials for the treatment of cancer. Here, we report the, first atomic structures of ONC-nucleic acid complexes: a T89N/E91A, ONC-5'-AMP complex at 1.65 A resolution and a wild-type ONC-d(AUGA), complex at 1.90 A resolution. The latter structure and site-directed, mutagenesis were used to reveal the atomic basis for substrate recognition, and turnover by ONC. The residues in ONC that are proximal to the scissile, phosphodiester bond (His10, Lys31, and His97) and uracil nucleobase, (Thr35, Asp67, and Phe98) are conserved from RNase A and serve to generate, a similar bell-shaped pH versus k(cat)/K(M) profile for RNA cleavage., Glu91 of ONC forms two hydrogen bonds with the guanine nucleobase in, d(AUGA), and Thr89 is in close proximity to that nucleobase. Installing a, neutral or cationic residue at position 91 or an asparagine residue at, position 89 virtually eliminated the 10(2)-fold guanine:adenine preference, of ONC. A variant that combined such substitutions, T89N/E91A ONC, actually preferred adenine over guanine. In contrast, installing an, arginine residue at position 91 increased the guanine preference and, afforded an ONC variant with the highest known k(cat)/K(M) value. These, data indicate that ONC discriminates between guanine and adenine by using, Coulombic interactions and a network of hydrogen bonds. The structure of, the ONC-d(AUGA) complex was also used to probe other aspects of catalysis., For example, the T5R substitution, designed to create a favorable, Coulombic interaction between ONC and a phosphoryl group in RNA, increased, ribonucleolytic activity by twofold. No variant, however, was more toxic, to human cancer cells than wild-type ONC. Together, these findings provide, a cynosure for understanding catalysis of RNA cleavage in a system of high, medicinal relevance.

About this Structure

2I5S is a Single protein structure of sequence from Rana pipiens. Full crystallographic information is available from OCA.

Reference

Structural basis for catalysis by onconase., Lee JE, Bae E, Bingman CA, Phillips GN Jr, Raines RT, J Mol Biol. 2008 Jan 4;375(1):165-77. Epub 2007 Oct 4. PMID:18001769

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