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Molecular and Cellular Biology, May 2002, p. 3194-3203, Vol. 22, No. 9
0270-7306/02/$04.00+0     DOI: 10.1128/MCB.22.9.3194-3203.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Assembly and Function of the RNA Editing Complex in Trypanosoma brucei Requires Band III Protein

Catherine E. Huang, Sean F. O'Hearn, and Barbara Sollner-Webb*

Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

Received 8 October 2001/ Returned for modification 20 November 2001/ Accepted 7 January 2002

Trypanosome RNA editing, the posttranscriptional insertion and deletion of U residues in mitochondrial transcripts, is catalyzed by a protein complex containing seven distinct proteins. In this study, we cloned the gene for band III, a 555-amino-acid protein with two separate zinc finger motifs. We prepared antibodies that showed band III protein cofractionates with the previously characterized band IV protein throughout the purification of the editing complex and is not found free or in other protein associations; therefore, it is a true constituent of the editing complex. Double-stranded RNA interference efficiently depleted band III protein and demonstrated that band III expression is essential for growth of procyclic trypanosomes and for RNA editing. These depleted cell extracts were deficient specifically in guide RNA-directed endonuclease cleavage at both U deletion and U insertion sites and in the activity of the band IV ligase, but they retained the 3'-U-exonuclease and terminal-U-transferase activities as well as band V ligase of the editing complex. Loss of band III protein also resulted in almost complete loss of the band IV ligase protein and altered sedimentation of the band V ligase. These data indicate that band III is either the RNA editing endonuclease or a factor critical for cleavage activity in the editing complex. They also demonstrate that band III is required for proper assembly of the editing complex.

* Corresponding author. Mailing address: Department of Biological Chemistry, Johns Hopkins University School of Medicine, 725 N. Wolfe Street, Baltimore, MD 21205. Phone (410) 955-6278. Fax: (410) 955-0192. E-mail: bsw{at}jhmi.edu.

Molecular and Cellular Biology, May 2002, p. 3194-3203, Vol. 22, No. 9
0022-538X/02/$04.00+0     DOI: 10.1128/MCB.22.9.3194-3203.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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