In Setaria italica (foxtail millet), SiUBC39 is implicated in regulating key agronomic traits, including plant height, flowering time, and stress tolerance. To elucidate the molecular mechanisms underlying SiUBC39’s roles in these traits, we affinity-purified SiUBC39-GFP and the corresponding GFP control protein via immunoprecipitation (IP). The immunoprecipitated complexes were then subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis to identify proteins interacting with SiUBC39.