{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Cytochrome c oxidase subunit III (COX3) is a mitochondrially encoded core component of complex IV that plays a central role in electron transport and ATP production. Several lines of evidence now indicate that COX3 function is regulated at multiple levels. Mitochondrial signaling pathways can influence its transcript levels because activation of a mitochondrial NF‐κB pathway leads to phosphorylation and degradation of IκBα and consequently to reduced expression of the COX3 mRNA along with other mitochondrial transcripts."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " Proper post‐transcriptional processing of the COX3 transcript also is critical; for example, a nuclear‐encoded mitochondrial poly(A) polymerase ensures that COX3 mRNAs receive correct polyadenylation, a process that influences mRNA stability even though shortened poly(A) tails may not dramatically destabilize the transcript."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Furthermore, studies on cytochrome c oxidase assembly demonstrate that COX3, together with other mtDNA‐encoded subunits, is assembled through discrete intermediates. Analysis of patient fibroblasts with mutations in key assembly factors revealed that COX3 is incorporated as part of a subassembly module that eventually associates with nuclear‐encoded subunits to form the mature enzyme."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": " This modular assembly process was further delineated using an MT‑CO3 mutant cybrid cell line that enabled the identification of novel assembly chaperones facilitating proper complex IV assembly."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In addition, defects in mitochondrial translation can adversely affect COX3 production. In models of mitochondrial disease, errors in translation fidelity have been linked to misincorporation of amino acids in the COX3 polypeptide, thereby contributing to defective respiratory chain function."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "5"}]}, {"type": "t", "text": " Alterations in COX3 expression have even been associated with metabolic adaptations in cancer; for instance, increased COX3 mRNA levels have been observed in chemoresistant acute myelogenous leukemia blasts."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Finally, evolutionary and regulatory studies underscore that COX3, although encoded by the mitochondrial genome, is subject to tight regulation by nuclear factors that modulate the efficiency and control of oxidative phosphorylation. These findings emphasize the pivotal role of COX3 in coordinating the integration of mitochondrial and nuclear signals to maintain cellular energy homeostasis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In summary, COX3 not only contributes structurally to the cytochrome c oxidase complex but also is dynamically regulated at the transcriptional, post‐transcriptional, translational, and assembly levels to ensure optimal mitochondrial respiratory function.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Patricia C Cogswell, David F Kashatus, Jayne A Keifer, et al. "}, {"type": "b", "children": [{"type": "t", "text": "NF-kappa B and I kappa B alpha are found in the mitochondria. 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