In most Actinobacteria, the respiratory complexes CIII and CIV form an obligate supercomplex, but the exact subunit composition varies. Here, we have characterized AscF (MSMEG_4692), a subunit of the Mycobacterium smegmatis CIII-CIV supercomplex. We showed that AscF and the small, membrane-anchored AscG constitute a heteromeric TPM domain featuring a noncanonical topology. Biophysical analysis demonstrated that the isolated AscF/AscG module lacked intrinsic affinity for metals or respiratory nucleotides in vitro. Functionally, an ascF frameshift mutant exhibited abolished malate-dependent oxygen consumption and severe growth defects on nonfermentable energy sources. We conclude that AscF likely is not a sensor for metal ions or nucleotides but acts as an adapter subunit facilitating electron transfer from the tricarboxylic acid cycle to the mycobacterial respiratory supercomplex.