SDH6 and SDH7 Contribute to Anchoring Succinate Dehydrogenase to the Inner Mitochondrial Membrane in Arabidopsis thaliana

Abstract

The succinate dehydrogenase complex (complex II) is a highly conserved protein complex composed of the SDH1 to SDH4 subunits in bacteria and in the mitochondria of animals and fungi. The reason for the occurrence of up to four additional subunits in complex II of plants, termed SDH5 to SDH8, so far is a mystery. Here, we present a biochemical approach to investigate the internal subunit arrangement of Arabidopsis (Arabidopsis thaliana) complex II. Using low-concentration detergent treatments, the holo complex is dissected into subcomplexes that are analyzed by a three-dimensional gel electrophoresis system. Protein identifications by mass spectrometry revealed that the largest subcomplex (IIa) represents the succinate dehydrogenase domain composed of SDH1 and SDH2. Another subcomplex (IIb) is composed of the SDH3, SDH4, SDH6, and SDH7 subunits. All four proteins include transmembrane helices and together form the membrane anchor of complex II. Sequence analysis revealed that SDH3 and SDH4 lack helices conserved in other organisms. Using homology modeling and phylogenetic analyses, we present evidence that SDH6 and SDH7 substitute missing sequence stretches of SDH3 and SDH4 in plants. Together with SDH5, which is liberated upon dissection of complex II into subcomplexes, SDH6 and SDH7 also add some hydrophilic mass to plant complex II, which possibly inserts further functions into this smallest protein complex of the oxidative phosphorylation system (which is not so small in plants).Succinate dehydrogenase (EC 1.3.5.1) is of central importance for energy metabolism in bacteria and mitochondria of eukaryotic cells. In mitochondria, it represents the complex II of the oxidative phosphorylation (OXPHOS) system (Hatefi, 1985) and is located in the inner mitochondrial membrane. Complex II participates in two major mitochondrial processes: the tricarboxylic acid cycle as well as the mitochondrial electron transfer chain (mETC). In the tricarboxylic acid cycle, it catalyzes the conversion of succinate into fumarate. Electrons originating from this reaction are inserted into the mETC and used for the reduction of ubiquinone to ubiquinol. In contrast to other protein complexes of the OXPHOS system, complex II does not contribute to the proton gradient across the inner mitochondrial membrane.The overall structure of the succinate dehydrogenase complex is remarkably conserved in bacteria, animals, and fungi, as revealed by biochemical investigations and x-ray crystallography (Yankovskaya et al., 2003;Oyedotun and Lemire, 2004;Sun et al., 2005;Huang et al., 2006;Iverson, 2013). It is about 120 kD in size and composed of four subunits designated SDH1, SDH2, SDH3, and SDH4 (also named SDHA to SDHD in some bacteria). SDH1 is the largest subunit and includes the succinate-binding site. Electrons from succinate are accepted by a covalently bound FAD group. SDH2 carries three iron-sulfur clusters that mediate electron transfer from SDH1 to the membrane domain of complex II. Together with SDH1, it constitutes the succinate dehydrog...

The mitochondrial complexome of Arabidopsis thaliana

“…Complex II (the succinate dehydrogenase complex) includes four subunits termed SDH1‐4 in most organisms investigated. In contrast, Arabidopsis complex II is composed of eight subunits (Eubel et al ., ; Millar et al ., ; Huang et al ., ; Schikowsky et al ., ). Seven of the eight complex II subunits form a cluster on the heat map of the Arabidopsis complexome (Figure , Appendix S1, p. 3).…”

Mitochondrial complex II of plants: subunit composition, assembly, and function in respiration and signaling

“…The subunit arrangement for G. gallus is taken from the crystal structure (Huang et al ., ); the arrangement for A. thaliana is based on the biochemical results presented by Schikowsky et al . (). The lower part of the complex, which protrudes into the mitochondrial matrix, includes SDH 1 and SDH 2.…”

“…Besides SDH 3 and SDH 4 it includes the plant‐specific subunits SDH 6 and SDH 7. SDH 7 is suggested to interact with SDH 3 and SDH 6 is suggested to interact with SDH 4 based on biochemical and evolutionary considerations (Schikowsky et al ., ). The localization of SDH 5 is probably at the interface between the succinate dehydrogenase and the membrane domain, but could also be found at another position.…”

“…In parallel with the transfer of SDH3 and SDH4 from the mitochondrial genome to the nuclear genome in some plants, SDH3 and SDH4 became truncated and more hydrophilic. Using sequence comparisons of SDH3 and SDH4 proteins from various plants it was shown that the truncation of SDH4 preceded the truncation of SDH3 during the evolution of Streptophyta (Embryophyta and Charophyta) (Schikowsky et al ., ).…”

“…Topological model of mitochondrial complex II of animal ( Gallus gallus ) (a) and plant ( Arabidopsis thaliana ) (b) (from Schikowsky et al ., ; modified). The subunit arrangement for G. gallus is taken from the crystal structure (Huang et al ., ); the arrangement for A. thaliana is based on the biochemical results presented by Schikowsky et al .…”