To identify the unique ubiquitin conjugation to Legionella pneumophila SdcB, we investigated the modified SdcB by IP-MS analysis. We revealed that a Legionella transglutaminase MavC has a catalytic activity to crosslink Gln residues of ubiquitin to Lys residues of SdcB, resulting in a noncanonical ubiquitin modification, and that this modification abolish the enzymatic activity of SdcB as a E3 ubiquitin ligase.