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Nucleic Acids Research Advance Access published online on August 28, 2008
Nucleic Acids Research, doi:10.1093/nar/gkn536
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© 2008 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

RNA

A mosaic of RNA binding and protein interaction motifs in a bifunctional mitochondrial tRNA import factor from Leishmania tropica

Pratik Home, Saikat Mukherjee and Samit Adhya*

Genetic Engineering Laboratory, Indian Institute of Chemical Biology, 4 Raja S. C. Mullick Road, Kolkata 700032, India

*To whom correspondence should be addressed. Tel: +91 33 2473 3491 X136; Fax: +91 33 2473 5197; Email: sadhya{at}iicb.res.in

Received June 24, 2008. Revised August 4, 2008. Accepted August 4, 2008.

Proteins that participate in the import of cytosolic tRNAs into mitochondria have been identified in several eukaryotic species, but the details of their interactions with tRNA and other proteins are unknown. In the kinetoplastid protozoon Leishmania tropica, multiple proteins are organized into a functional import complex. RIC8A, a tRNA-binding subunit of this complex, has a C-terminal domain that functions as subunit 6b of ubiquinol cytochrome c reductase (complex III). We show that the N-terminal domain, unique to kinetoplastid protozoa, is structurally similar to the appended S15/NS1 RNA-binding domain of aminoacyl tRNA synthetases, with a helix–turn–helix motif. Structure-guided mutagenesis coupled with in vitro assays showed that helix {alpha}1 contacts tRNA whereas helix {alpha}2 targets the protein for assembly into the import complex. Inducible expression of a helix 1-deleted variant in L. tropica resulted in formation of an inactive import complex, while the helix 2-deleted variant was unable to assemble in vivo. Moreover, a protein-interaction assay showed that the C-terminal domain makes allosteric contacts with import receptor RIC1 complexed with tRNA. These results help explain the origin of the bifunctionality of RIC8A, and the allosteric changes accompanying docking and release of tRNA during import.


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