Mitochondrial respiratory chain (MRC) enzymes associate in supercomplexes (SCs) that are structurally interdependent. This may explain why defects in a single component often produce combined enzyme deficiencies in patients. A case in point is the alleged destabilization of complex I in the absence of complex III. To clarify the structural and functional relationships between complexes, we have used comprehensive proteomic, functional, and biogenetical approaches to analyze a MT-CYB-deficient human cell line. We show that the absence of complex III blocks complex I biogenesis by preventing the incorporation of the NADH module rather than decreasing its stability. In addition, complex IV subunits appeared sequestered within complex III subassemblies, leading to defective complex IV assembly as well. Therefore, we propose that complex III is central for MRC maturation and SC formation. Our results challenge the notion that SC biogenesis requires the pre-formation of fully assembled individual complexes. In contrast, they support a cooperative-assembly model in which the main role of complex III in SCs is to provide a structural and functional platform for the completion of overall MRC biogenesis.
Summary
Mitochondrial respiratory chain (MRC) complexes I, III and IV associate into a variety of supramolecular structures known as supercomplexes and respirasomes. While COX7A2L was originally described as a supercomplex-specific factor responsible for the dynamic association of complex IV into these structures to adapt MRC function to metabolic variations, this role has been disputed. Here we further examine the functional significance of COX7A2L in the structural organization of the mammalian respiratory chain. As in the mouse, human COX7A2L binds primarily to free mitochondrial complex III and to a minor extent to complex IV to specifically promote the stabilization of the III2+IV supercomplex without affecting respirasome formation. Furthermore, COX7A2L does not affect the biogenesis, stabilization and function of the individual OXPHOS complexes. These data show that independent regulatory mechanisms for the biogenesis and turnover of different MRC supercomplex structures co-exist.
Obesity is increasing exponentially in developed countries and constitutes a public health problem by enhancing the risk for metabolic disorder and cardiovascular disease. Differences in gene expression profiles and in metabolic and biochemical properties have been well-described between omental and subcutaneous adipose tissue in humans. Because omental adipose tissue has been strongly associated with the development of insulin resistance, type 2 diabetes and cardiovascular disease, we searched for proteins differentially expressed in these two fat depots using two-dimensional fluorescence difference gel electrophoresis (2D-DIGE) and mass spectrometry (MS). In this analysis, we found 43 proteins, several of which were validated by immunoblotting and immunostaining analyses. Results demonstrated tissue-specific molecular differences in the protein makeup of the two analyzed fat depots mainly related to metabolic processes such as glucose and lipid metabolism, lipid transport, protein synthesis, protein folding, response to stress and inflammation. This suggests higher metabolic activity as well as increased cell stress in the omental compared to the subcutaneous fat. These findings provide some insights into the role of omental fat in abdominal obesity-associated co-morbidities.
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