The microprotein SMIM26 connects metabolite transporters of the outer and inner mitochondrial membranes and is essential for respiratory chain function.

Microproteins represent a class of short polypeptides with very diverse cellular functions. Microproteins frequently escape proteomics-based identification, making the extent and potential functions of small proteins largely elusive. Some microproteins originate from transcripts that are annotated as long noncoding RNAs (lncRNAs). Here, we functionally characterize SMIM26, a microprotein localized to mitochondria. In biochemical and single-molecule tracking studies, we found that SMIM26 interacts with VDAC1/2 in the outer mitochondrial membrane and with SLC25A6 in the inner mitochondrial membrane. It spans the intermembrane space and is phosphorylated at distinct residues. Knockout cells are viable, but respiratory chain activity is strongly reduced. Interestingly, knockout mice are not viable and die at early developmental stages. Zebrafish homozygous smim26 mutants are viable but show reduced fitness and survival compared with their wild-type or heterozygous siblings. Consistent with the mitochondrial phenotype in cell lines, respiration is also reduced in homozygous zebrafish embryos. Our work suggests that SMIM26 coordinates metabolite transport through the inner and outer mitochondrial membranes and is essential for respiratory chain function in vivo.