| Methicillin-resistant Staphylococcus aureus(MRSA)is a common clinical pathogen that leads to difficult-to-treat,in many cases even fatal,infections to human.Most MRSA stains are resistant to all ?-lactams due to the abuse of antibiotics in recent decades.As a main component of the cell wall of MRSA,the wall teichoic acid(WTA)plays a key role in the resistance of antibiotics.WTA is an anionic glycopolymer covalently attaching to the peptidoglycan of Gram-positive bacteria and is crucial for the cell division,biofilm formation,host colonization and infection,thus making itself a promising antigen and the biosynthesis pathway a potential target for developing new antimicrobials.In Staphylococcus aureus,biosynthesis of the WTA precursor,an N-acetylglucosamine modified ribitol-phosphate polymer covalently linked to the UND-PP moiety,is accomplished in the cytoplasm.The WTA ABC transporter TarGH is finally responsible for the transmembrane flipping of WTA precursor out of the cell membrane,thus making this transporter a potential target of antibiotics-Recently,Targocil,an inhibitor targeted to TarGH has been reported,but its inhibitory mechanism remains unclear.Thus the structure of TarGH is needed to elucidate the transport mechanism of TarGH as well as the inhibitory mechanism of Targocil.Here we report the 3.9 A cryo-EM structure of a 50%sequence-identical homolog of S.aureus TarGH from Alicyclobacillus herbarius at an ATP-free and inward-facing conformation.Structural analyses reveal a well-defined tunnel for the substrate and enable us to propose a "crankshaft conrod" mechanism that translocates a rather big substrate through relatively subtle conformational changes.This mechanism can be applied to ABC transporters that possess a similar structure.Moreover,we clearly decode the binding site and inhibitory mechanism of the anti-MRSA inhibitor Targocil that targets to TarGH.These findings provide structural basis for the rational design and optimization of antibiotics against MRSA. |
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