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First published online July 22, 2005; 10.1104/pp.105.062745 Plant Physiology 138:2134-2144 (2005) © 2005 American Society of Plant Biologists Adaptations Required for Mitochondrial Import following Mitochondrial to Nucleus Gene Transfer of Ribosomal Protein S101,[w]Australian Research Council Centre of Excellence in Plant Energy Biology, University of Western Australia, Crawley 6009, Western Australia (M.W.M., D.E., J.W.); Department of Biochemistry and Biophysics, Stockholm University, SE–106 91, Sweden (C.R., D.O.D.); and University of British Columbia Botanical Garden and Centre for Plant Research, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada (K.L.A.)
The minimal requirements to support protein import into mitochondria were investigated in the context of the phenomenon of ongoing gene transfer from the mitochondrion to the nucleus in plants. Ribosomal protein 10 of the small subunit is encoded in the mitochondrion in soybean and many other angiosperms, whereas in several other species it is nuclear encoded and thus must be imported into the mitochondrial matrix to function. When encoded by the nuclear genome, it has adopted different strategies for mitochondrial targeting and import. In lettuce (Lactuca sativa) and carrot (Daucus carota), Rps10 independently gained different N-terminal extensions from other genes, following transfer to the nucleus. (The designation of Rps10 follows the following convention. The gene is indicated in italics. If encoded in the mitochondrion, it is rps10; if encoded in the nucleus, it is Rps10.) Here, we show that the N-terminal extensions of Rps10 in lettuce and carrot are both essential for mitochondrial import. In maize (Zea mays), Rps10 has not acquired an extension upon transfer but can be readily imported into mitochondria. Deletion analysis located the mitochondrial targeting region to the first 20 amino acids. Using site directed mutagenesis, we changed residues in the first 20 amino acids of the mitochondrial encoded soybean (Glycine max) rps10 to the corresponding amino acids in the nuclear encoded maize Rps10 until import was achieved. Changes were required that altered charge, hydrophobicity, predicted ability to form an amphiphatic
1 This work was supported by the Australian Research Council (to J.W.). [w] The online version of this article contains Web-only data. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.062745. * Corresponding author; e-mail seamus{at}cyllene.uwa.edu.au; fax 61–(0)8–64881148. Received March 12, 2005; returned for revision May 5, 2005; accepted May 16, 2005. This article has been cited by other articles:
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-helix, and generation of a binding motif for the outer mitochondrial membrane receptor, translocase of the outer membrane 20. In addition to defining the changes required to achieve mitochondrial localization, the results demonstrate that even proteins that do not present barriers to import can require substantial changes to acquire a mitochondrial targeting signal. 
