Small proteins in prokaryotes are usually defined as polypeptides that are shorter than 80 amino acids. In the model cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803), the existence of several small proteins was verified previously. Among them, Norf1 (renamed to AtpΘ afterwards) is a small protein that is highly accumulated in darkness or stationary phase. To clarify the functions of AtpΘ, co-immunoprecipitation was performed to identify its interaction partners using FLAG-tagged AtpΘ protein as bait and FLAG-tagged sfGFP as negative control in three biological replicates each.