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MBE Advance Access published online on June 16, 2009
Molecular Biology and Evolution, doi:10.1093/molbev/msp117
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© The Author 2009. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Research Article

The protein import channel in the outer mitosomal membrane of Giardia intestinalis

Michael J. Dagley1,2, Pavel Dolezal3, Vladimir A. Likic2, Ondrej Smid3, Anthony W. Purcell1,2, Susan K. Buchanan4, Jan Tachezy3 and Trevor Lithgow1,5,*

1 Department of Biochemistry and Molecular Biology, University of Melbourne, Parkville 3010, Australia
2 Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, Parkville 3010, Australia
3 Department of Parasitology, Faculty of Science, Charles University, Vinicna 7, Praha 2, 128 44, Czech Republic
4 Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, US National Institutes of Health, Bethesda, Maryland 20892, USA
5 ARC Federation Fellow, Department of Biochemistry & Molecular Biology, Monash University, Clayton campus, Wellington Road, Clayton 3800, Australia

* To whom correspondence should be addressed: Professor Trevor Lithgow, Department of Biochemistry & Molecular Biology, Monash University, Clayton campus, Wellington Road, Clayton 3800, Australia. Phone: +61 3 9902 9217. E-mail: trevor.lithgow{at}med.monash.edu.au

Received for publication May 27, 2009. Accepted for publication June 8, 2009.

The identification of mitosomes in Giardia generated significant debate on the evolutionary origin of these organelles; whether they were highly reduced mitochondria, or the product of a unique endosymbiotic event in an amitochondrial organism. As the protein import pathway is a defining characteristic of mitochondria, we sought to discover a TOM (translocase in the outer mitochondrial membrane) complex in Giardia. A Hidden Markov model search of the Giardia genome identified a Tom40 homologous sequence (GiTom40), where Tom40 is the protein translocation channel of the TOM complex. The GiTom40 protein is located in the membrane of mitosomes in a ~200 kDa TOM complex. As Tom40 was derived in the development of mitochondria to serve as the protein import channel in the outer membrane, its presence in Giardia evidences the mitochondrial ancestry of mitosomes.

Key Words: mitochondria • mitosomes • Giardia intestinalis • protein translocation • evolution


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