Discovery and structure of a widespread bacterial ABC transporter specific for ergothioneine

Abstract

ABSTRACTErgothioneine (ET) is the 2-thiourea derivative of trimethylhistidine that is biosynthesized only by select fungi and bacteria, notablyMycobacterium tuberculosis, and functions as a potent scavenger of reactive oxygen species. Although ET is obtained in the diet and accumulates in vertebrate cells via an ET-specific transporter, the extent to which ET broadly functions in bacterial cells unable to synthesize it is unknown. Here we show thatspd_1642-1643inStreptococcus pneumoniaeD39, a Gram-positive respiratory pathogen, encodes a novel ergothioneine uptake ATP-binding cassette (ABC) transporter, which we designate EgtUV. EgtU is a permease-solute binding domain (SBD) fusion protein, and the SBD binds ET with high affinity and exquisite specificity in the cleft between the two subdomains, with cation-π interactions engaging the betaine moiety and a water-mediated hydrogen bonding network surrounding the C2-sulfur-containing imidazole ring. Bioinformatics studies reveal that EgtUV is uniquely strongly conserved among known quaternary amine-specific transporters and widely distributed in firmicutes, including the human pathogensListeria monocytogenes, as BilEB,Enterococcus faecalisandStaphylococcus aureus. This discovery significantly diversifies the LMW thiol pool in Gram-positive human pathogens that may contribute to antioxidant defenses in the infected host.