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Serine/threonine phosphatase (Stp1) is a member of the bacterial Mg2+-or Mn2+-dependent protein phosphatase/protein phosphatase 2C family,which is involved in the regulation of Staphylococcus aureus virulence.Aurintricarboxylic acid (ATA) is a known Stp1 inhibitor with an IC50 of 1.03 μM,but its inhibitory mechanism has not been elucidated in detail because the Stp1-ATA cocrystal structure has not been determined thus far.In this study,we performed 400 ns molecular dynamics (MD) simulations of the apo-Stp1 and Stp1-ATA complex models.During MD simulations,the flap subdomain of the Stp1-ATA complex experienced a clear conformational transition from an open state to a closed state,whereas the flap domain of apo-Stp1 changed from an open state to a semi-open state.In the Stp1-ATA complex model,the hydrogen bond (H-bond) between D137 and N142 disappeared,whereas critical H-bond interactions were formed between Q160 and H13,Q160/R161 and ATA,as well as N 162 and D198.Finally,four residues (D137,N142,Q160,and R161) in Stp1 were mutated to alanine and the mutant enzymes were assessed using phosphate enzyme activity assays,which confirmed their important roles in maintaining Stp1 activity.This study indicated the inhibitory mechanism of ATA targeting Stp1 using MD simulations and sheds light on the future design of allosteric Stp1 inhibitors.