The two roles of complex III in plants

Atomic structures of mitochondrial enzyme complexes in plants are shedding light on their multiple functions.

A conserved mammalian mitochondrial isoform of acetyl-CoA carboxylase ACC1 provides the malonyl-CoA essential for mitochondrial biogenesis in tandem with ACSF3

Mitochondrial fatty acid synthesis (mtFAS) is a highly conserved pathway essential for mitochondrial biogenesis. The mtFAS process is required for mitochondrial respiratory chain assembly and function, synthesis of the lipoic acid cofactor indispensable for the function of several mitochondrial enzyme complexes and essential for embryonic development in mice. Mutations in human mtFAS have been reported to lead to neurodegenerative disease. The source of malonyl-CoA for mtFAS in mammals has remained unclear. We report the identification of a conserved vertebrate mitochondrial isoform of ACC1 expressed from an ACACA transcript splicing variant. A specific knockdown (KD) of the corresponding transcript in mouse cells, or CRISPR/Cas9-mediated inactivation of the putative mitochondrial targeting sequence in human cells, leads to decreased lipoylation and mitochondrial fragmentation. Simultaneous KD of ACSF3, encoding a mitochondrial malonyl-CoA synthetase previously implicated in the mtFAS process, resulted in almost complete ablation of protein lipoylation, indicating that these enzymes have a redundant function in mtFAS. The discovery of a mitochondrial isoform of ACC1 required for lipoic acid synthesis has intriguing consequences for our understanding of mitochondrial disorders, metabolic regulation of mitochondrial biogenesis and cancer.

Characterization by X-ray powder diffraction of alpha lipoic acid

Alpha lipoic acid (ALA) C8H14O2S2 is a naturally occurring compound that is synthesized in small amounts by plants and animals, including humans. ALA is covalently bound to specific proteins, which function as cofactors for several important mitochondrial enzyme complexes and studies suggest that they might help with type 2 diabetes. In the Cambridge Structural Database, there are four entries related to this compound: two for lipoic acid and two for complexes. In the Powder Diffraction File-4, two experimental unindexed patterns are reported. The material crystallizes in a monoclinic crystal system, space group P21/a and cell parameters a = 9.237 (1) Å, b = 9.960 (1) Å, c = 11.787 (2) Å, β = 109.13 (1)°, and V = 1024.6 (2) Å3.

Function of the maize mitochondrial chaperonin hsp60: specific association between hsp60 and newly synthesized F1-ATPase alpha subunits.

Mitochondria contain a protein, hsp60, that is induced by heat shock and has been shown to function as a chaperonin in the assembly of mitochondrial enzyme complexes composed of proteins encoded by nuclear genes and imported from the cytosol. To determine whether products of mitochondrial genes are also assembled through an interaction with hsp60, we looked for association between hsp60 and proteins synthesized by isolated mitochondria. We have determined by electrophoretic, centrifugal, and immunological assays that at least two of those proteins become physically associated with hsp60. In mitochondrial matrix extracts, this association could be disrupted by the addition of Mg-ATP. One of the proteins that formed a stable association with hsp60 was the alpha subunit of the multicomponent complex F1-ATPase. We have not identified the other protein. These results indicate that hsp60 can function in the folding and assembly of mitochondrial proteins encoded by both mitochondrial and nuclear genes.

Function of the maize mitochondrial chaperonin hsp60: specific association between hsp60 and newly synthesized F1-ATPase alpha subunits

Mitochondria contain a protein, hsp60, that is induced by heat shock and has been shown to function as a chaperonin in the assembly of mitochondrial enzyme complexes composed of proteins encoded by nuclear genes and imported from the cytosol. To determine whether products of mitochondrial genes are also assembled through an interaction with hsp60, we looked for association between hsp60 and proteins synthesized by isolated mitochondria. We have determined by electrophoretic, centrifugal, and immunological assays that at least two of those proteins become physically associated with hsp60. In mitochondrial matrix extracts, this association could be disrupted by the addition of Mg-ATP. One of the proteins that formed a stable association with hsp60 was the alpha subunit of the multicomponent complex F1-ATPase. We have not identified the other protein. These results indicate that hsp60 can function in the folding and assembly of mitochondrial proteins encoded by both mitochondrial and nuclear genes.