H

Small open reading frame (sORF)-encoded proteins, with less than 100 amino acids, have attracted increasing attention over the past decade after being overlooked due to limitations in classical methodologies. For the mesophilic archaeal model system Methanosarcina mazei, a high number of previously unannotated sORFs have recently been identified. However, the physiological role of most of the respective small proteins remains unknown. Here, we report on the functional characterization of the small ORF16-encoded small protein MtrR (49 amino acids). We demonstrate that MtrR forms oligomers localized at the cytoplasmic membrane. There, it interacts with the tetrahydrosarcinapterin S-methyltransferase (Mtr), a key membrane-bound complex of energy metabolism, and impacts its activity. In vitro interaction and in vivo copurification assays showed MtrR interaction with the Mtr complex, which was further validated by microscale thermophoresis analysis demonstrating a specific interaction with the MtrA subunit. Analyzing growth under varying molecular hydrogen (H