Abstract
Complex I is present in almost all aerobic species. Being the largest complex of the respiratory chain, it has a central role in energizing biological membranes and is essential for many organisms. Bacterial complex I is composed of 14 subunits that are sufficient to achieve the respiratory functions. Eukaryotic enzymes contain orthologs of the 14 bacterial subunits and around 30 additional subunits. This complexity suggests either that complex I requires more stabilizing subunits in mitochondria or that it fulfills additional functions. In many organisms recent work on complex I concentrated on the determination of its exact composition. This review summarizes the work done to elucidate complex I composition in the model plant Arabidopsis and proposes a model for the organization of its 44 confirmed subunits. The comparison of the different studies investigating the composition of complex I across species identifies sample preparation for the proteomic analysis as critical to differentiate between true subunits, assembly factors, or proteins associated with complex I. Coupling comparative proteomics with biochemical or genetic studies is thus required to define a subunit and its function within the complex.
Highlights
The NADH-Ubiquinone oxidoreductase (EC 1.6.5.3) is part of the respiratory chain present in all aerobic organisms
More than 15 years after the first efforts to determine the subunit composition of complex I in plants, 44 confirmed complex I subunits are known in the model organism Arabidopsis thaliana
Knowledge of the composition of complex I is a prerequisite for defining strategies to characterize the functions that complex I fulfills in plant mitochondria
Summary
The NADH-Ubiquinone oxidoreductase (EC 1.6.5.3) is part of the respiratory chain present in all aerobic organisms. The first proteomic analysis of Arabidopsis and rice complex I coupling BN-SDS-PAGE and mass spectrometry (MS) identified 30 and 24 proteins, respectively (Heazlewood et al, 2003) These proteins comprised several plant-specific subunits, defined as having no homologs among the subunits of complex I from bovine or Neurospora. Among the Arabidopsis CAs, two carbonic anhydrase-like (CAL) proteins have been identified and were considered to represent two distinct complex I subunits Other higher plants, such as rice, maize, or poplar, possess only one gene encoding CAL. TIM22 (encoded by At1g18320 and At3g10110) has been identified in the vertical containing complex I subunits on a BN-SDS-PAGE (Klodmann et al, 2011) As this protein acts as an import channel during the import of cytoplasmically translated carriers into the inner mitochondrial membrane (Murcha et al, 2007), a role of TIM22 in the assembly of complex I is plausible. More biochemical evidence is required to validate these proteins as complex I subunits
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