The structure of NAD plus consuming protein Acinetobacter baumannii TIR domain shows unique kinetics and conformations

Toll-like and interleukin-1/18 receptor/resistance (TIR) domain-containing proteins function as important signaling and immune regulatory molecules. TIR domain-containing proteins identified in eukaryotic and prokaryotic species also exhibit NAD+ hydrolase activity in select bacteria, plants, and mammalian cells. We report the crystal structure of the Acinetobacter baumannii TIR domain protein (AbTir-TIR) with confirmed NAD+ hydrolysis and map the conformational effects of its interaction with NAD+ using hydrogen-deuterium ex-change-mass spectrometry. NAD+ results in mild decreases in deuterium uptake at the dimeric interface. In addition, AbTir-TIR exhibits EX1 kinetics indicative of large cooperative conformational changes, which are slowed down upon substrate binding. Additionally, we have developed label-free imaging using the minimally invasive spectroscopic method 2-photon excitation with fluorescence lifetime imaging, which shows differences in bacteria expressing native and mutant NAD+ hydrolase-inactivated AbTir-TIRE208A protein. Our observations are consistent with substrate-induced conformational changes reported in other TIR model systems with NAD+ hydrolase activity. These studies provide further insight into bacterial TIR protein mechanisms and their varying roles in biology.