New insights into the organisation of the oxidative phosphorylation system in the example of pea shoot mitochondria.

The physical and functional organisation of the OXPHOS system in mitochondria in vivo remains elusive. At present, different models of OXPHOS arrangement, representing either highly ordered respiratory strings or, vice versa, a set of randomly dispersed supercomplexes and respiratory complexes, have been suggested. In the present study, we examined a supramolecular arrangement of the OXPHOS system in pea shoot mitochondria using digitonin solubilisation of its constituents, which were further analysed by classical BN-related techniques and a multidimensional gel electrophoresis system when required. As a result, in addition to supercomplexes IIII, IIIIIV and IIIIV, dimer V, and individual complexes I-V previously detected in plant mitochondria, new OXPHOS structures were also revealed. Of them, (1) a megacomplex (IIIIIIV)n including complex II, (2) respirasomes IIIIIV with two copies of complex I and dimeric complex III, (3) a minor new supercomplex IVVa comigrating with IIII, and (4) a second minor form of ATP synthase, Va, were found. The activity of singular complexes I, IV, and V was higher than the activity of the associated forms. The detection of new supercomplex IVVa, along with assemblies IIII and IIIIIV, prompted us to suggest the occurrence of in vivo oxphosomes comprising complexes I, III, IV, and V. The putative oxphosome's stoichiometry, historical background, assumed functional significance, and subcompartmental location are discussed herein.