Determining Expression, Regulation, and Stoichiometries of Cytochrome C Oxidase Subunit IV Paralogs in Fish

Abstract

Cytochrome c oxidase (COX) is complex IV of the electron transport chain and catalyzes the reduction of oxygen to water. It also simultaneously translocates protons from the mitochondrial matrix to the inter-membrane space, creating the driving force of ATP synthesis. The COX holoenzyme is made of ten nuclear-encoded subunits, the largest of which is COX4. The tissue distribution of the COX4 paralogs was expected to be similar in mammals and teleosts, with a ubiquitously transcribed COX4-1 and COX4-2 expression more abundant in brain and respiratory tissue. However, I found that in general, teleosts appear to have a greater expression of COX4-2 across all tissues compared to mammals. Where interspecies differences were observed in the relative abundance of COX4-2, the differences were not readily attributable to phylogeny or hypoxia-tolerance. Additionally, I looked at hypoxia responses of COX4 on tilapia (Oreochromis niloticus) because hypoxia stimulates transcription of COX4-2 in mammalian tissues. I found that neither COX4-2 nor the transcription factors that regulate COX4-2 in mammals (RBPJ, CHCHD2, and CXXC5) respond to physiological hypoxia. Finally, investigating the relationship between mRNA and protein, it appeared that protein is well predicted by mRNA at normoxic and hypoxic levels, with the exception of heart tissue (70% relative COX4-2 mRNA and 99% relative COX4-2 protein). Overall, the evidence suggests that although COX4-2 plays a more constitutive role in fish, it may not respond to hypoxia the same way it does in mammalian tissues.