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Peptidoglycan and virtually all surface glycopolymers in bacteria are built in the cytoplasm on the lipid carrier undecaprenyl-phosphate (UndP)1–4. These UndP-linked precursors are transported across the membrane and polymerized or directly transferred to surface polymers, lipids, or proteins. UndP is then flipped to regenerate the pool of cytoplasmic-facing UndP. The identity of the flippase that catalyzes transport has eluded identification for decades. Here, using the antibiotic amphomycin that targets UndP5–7, we discovered two broadly conserved protein families that affect UndP recycling. One (UptA) is a member of the DedA superfamily8; the other (PopT) contains the domain DUF368. Genetic, cytological, and syntenic analyses argue that these proteins are the missing UndP transporters. Importantly, homologs from gram-positive and gram-negative bacteria promote UndP transport in Bacillus subtilis, indicating that recycling activity is broadly conserved among family members. Inhibitors of these flippases could potentiate the current arsenal of cell envelope-targeting antibiotics.

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