MTALTCO1: a 259 amino acid long mtDNA-encoded alternative protein that challenges conventional understandings of mitochondrial genomics.

Mitochondrial alternative Open Reading Frames (ORFs) substantially broaden the functional scope traditionally attributed to mitochondrial DNA, encoding peptides and proteins that participate in diverse cellular processes. These newly identified ORFs are embedded within annotated sequences, both coding and non-coding, and reveal layers of overlapping genetic information. We here report the discovery of MTALTCO1, a 259 amino-acid protein, the longest mitochondrial alternative protein identified to date, encoded by an ORF located within the human cytochrome oxidase 1 gene, in the +3 reading frame. We confirm the expression and mitochondrial origin of MTALTCO1 through multiple independent lines of evidence, including a custom-designed antibody, mass spectrometry-derived peptides, sequence analysis, and inhibitors of mitochondrial expression. Despite encoding AGR codons as arginine, contrary to the prevailing view that these function as stop codons in the vertebrate mitochondrial genetic code, MTALTCO1 shows strong evidence of mitochondrial translation, challenging established models of mitochondrial codon usage and gene expression. Co-immunoprecipitations and pull-down assays delineate MTALTCO1s interaction landscape across major cellular pathways. Lastly, we present the first in-depth analysis of conservation for a mitochondrial alternative ORF overlapping a reference protein-coding gene and discuss the results in light of MTALTCO1s suggested role in protein scaffolding.