Plant Physiology Preview
Published on February 6, 2003; 10.1104/pp.102.015081
Received September 23, 2002
Returned for revision November 11, 2002
Accepted December 27, 2002
Dual Intracellular Localization and Targeting of Aminoimidazole Ribonucleotide Synthetase in Cowpea
Danica Erin Goggin , Richard Lipscombe , Elena Fedorova , A. Harvey Millar , Anthea Mann , Craig Anthony Atkins *, and Penelope Mary Collina Smith
Department of Botany (D.E.G., E.F., A.M., C.A.A., P.M.C.S.) and Biochemistry and Molecular Biology (A.H.M.), The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia; and Proteomics International Pty Ltd, Level 21, 197 St Georges Terrace, Perth, Western Australia 6000, Australia (R.L.)
* Corresponding author; email: catkins{at}cyllene.uwa.edu.au.
De novo purine biosynthesis is localized to both mitochondria and plastids isolated from Bradyrhizobium sp.-infected cells of cowpea (Vigna unguiculata L. Walp) nodules, but several of the pathway enzymes, including aminoimidazole ribonucleotide synthetase (AIRS [EC 6.3.3.1], encoded by Vupur5), are encoded by single genes. Immunolocalization confirmed the presence of AIRS protein in both organelles. Enzymatically active AIRS was purified separately from nodule mitochondria and plastids. N-terminal sequencing showed that these two isoforms matched the Vupur5 cDNA sequence but were processed at different sites following import; the mitochondrial isoform was five amino acids longer than the plastid isoform. Electrospray tandem mass spectrometry of a trypsin digest of mitochondrial AIRS identified two internal peptides identical with the amino acid sequence deduced from Vupur5 cDNA. Western blots of proteins from mitochondria and plastids isolated from root tips showed a single AIRS protein present at low levels in both organelles. 35S-AIRS protein translated from a Vupur5 cDNA was imported into isolated pea (Pisum sativum) leaf chloroplasts in vitro by an ATP-dependent process but not into import-competent mitochondria from several plant and non-plant sources. Components of the mature protein are likely to be important for import because the N-terminal targeting sequence was unable to target green fluorescent protein to either chloroplasts or mitochondria in Arabidopsis leaves. The data confirm localization of the protein translated from the AIRS gene in cowpea to both plastids and mitochondria and that it is cotargeted to both organelles, but the mechanism underlying import into mitochondria has features that are yet to be identified.
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