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J. Biol. Chem., Vol. 280, Issue 46, 38870-38878, November 18, 2005

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A Three-dimensional Working Model of the Multienzyme Complex of Aminoacyl-tRNA Synthetases Based on Electron Microscopic Placements of tRNA and Proteins^*

Cindy L. Wolfe1, J. Anthony Warrington, Lauren Treadwell, and Mona Trempe Norcum12

From the Department of Biology, Tougaloo College, Tougaloo, Mississippi 39174 and theDepartment of Biochemistry, The University of Mississippi Medical Center, Jackson, Mississippi 39216

It has become evident that the process of protein synthesis is performed by many cellular polypeptides acting in concert within the structural confines of protein complexes. In multicellular eukaryotes, one of these assemblies is a multienzyme complex composed of eight proteins that have aminoacyl-tRNA synthetase activities as well as three non-synthetase proteins (p43, p38, and p18) with diverse functions. This study uses electron microscopy and three-dimensional reconstruction to explore the arrangement of proteins and tRNA substrates within this "core" multisynthetase complex. Binding of unfractionated tRNA establishes that these molecules are widely distributed on the exterior of the structure. Binding of gold-labeled tRNA^Leu places leucyl-tRNA synthetase and the bifunctional glutamyl-/prolyl-tRNA synthetase at the base of this asymmetric "V"-shaped particle. A stable cell line has been produced that incorporates hexahistidine-labeled p43 into the multisynthetase complex. Using a gold-labeled nickel-nitrilotriacetic acid probe, the polypeptides of the p43 dimer have been located along one face of the particle. The results of this and previous studies are combined into an initial three-dimensional working model of the multisynthetase complex. This is the first conceptualization of how the protein constituents and tRNA substrates are arrayed within the structural confines of this multiprotein assembly.

Received for publication, March 14, 2005 , and in revised form, September 6, 2005.

^* This work was supported by National Science Foundation Grants MCB-0090539 (to M. T. N.) and MCB-0215940 (to C. L. W. and M. T. N.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Both authors contributed equally to this work.

² To whom correspondence should be addressed: Dept. of Biochemistry, The University of Mississippi Medical Center, 2500 North State St., Jackson, MS 39216-4505. Tel.: 601-984-1527; Fax: 601-984-1855, E-mail: mnorcum{at}biochem.umsmed.edu.

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