The Mitochondrial Complexome of Medicago truncatula

Abstract

Legumes (Fabaceae, Leguminosae) are unique in their ability to carry out an elaborate endosymbiotic nitrogen fixation process with rhizobia proteobacteria. The symbiotic nitrogen fixation enables the host plants to grow almost independently of any other nitrogen source. Establishment of symbiosis requires adaptations of the host cellular metabolism, here foremost of the energy metabolism mainly taking place in mitochondria. Since the early 1990s, the galegoid legume Medicago truncatula Gaertn. is a well-established model for studying legume biology, but little is known about the protein complement of mitochondria from this species. An initial characterization of the mitochondrial proteome of M. truncatula (Jemalong A17) was published recently. In the frame of this study, mitochondrial protein complexes were characterized using Two-dimensional (2D) Blue native (BN)/SDS-PAGE. From 139 detected spots, the "first hit" (=most abundant) proteins of 59 spots were identified by mass spectrometry. Here, we present a comprehensive analysis of the mitochondrial "complexome" (the "protein complex proteome") of M. truncatula via 2D BN/SDS-PAGE in combination with highly sensitive MS protein identification. In total, 1,485 proteins were identified within 158 gel spots, representing 467 unique proteins. Data evaluation by the novel GelMap annotation tool allowed recognition of protein complexes of low abundance. Overall, at least 36 mitochondrial protein complexes were found. To our knowledge several of these complexes were described for the first time in Medicago. The data set is accessible under http://www.gelmap.de/medicago/. The mitochondrial protein complex proteomes of Arabidopsis available at http://www.gelmap.de/arabidopsis/ and Medicago are compared.

Keywords: 2D BN/SDS-PAGE; GelMap annotation tool; Medicago truncatula; mitochondrial complexome; mitochondrial prohibitins.

Figure 1

GelMap reference map of the M. truncatula mitochondrial protein complexe proteome/complexome (http://www.gelmap.de/medicago/). Hundred and fifty-eight protein spots separated by 2D BN/SDS-PAGE and identified by MS are marked by circles. Most protein spots include multiple protein annotations. By clicking a certain protein spot, all identified proteins within this spot are shown in a pop-up window, beginning with the protein identification of the highest MASCOT score. The menu to the right lists classes of physiological functions for mitochondrial protein complexes. By clicking on the selected protein complex in this menu, accessions of all included individual proteins as well as the corresponding protein spots in the gel image are highlighted. Alternatively, a protein can be found in GelMap by the Search tool at the bottom to the right.

Figure 2

Detailed protein complex visualization in the GelMap reference maps of the Medicago mitochondrial complexome (http://www.gelmap.de/medicago/, left row) as compared to the Arabidopsis mitochondrial complexome (http://www.gelmap.de/arabidopsis/, right row). Visualization of protein complexes and all of its included individual proteins is exemplarily done for: (A) external/alternative enzymes, (B) other transporters/ABC transporters, and for (C) prohibitin complexes.