3FNC1†These authors contributed equally to this work.
3FNC2‡Present address: Children's Hospital and Research Institute ‘Bambino Gesù’, Piazza S. Onofrio 4, I-00165 Rome, Italy.
3FNC§Present address: National Research Council, Istituto di Cristallografia, Sezione di Monterotondo, Monterotondo Stazione, I-00016 Rome, Italy.
3FNC4||Present address: Department of Experimental Medicine and Biochemical Sciences, University of Rome ‘Tor Vergata’, Via Montpellier 1, I-001 Rome, Italy.
The l-ornithine Nδ-oxygenase PvdA catalyses the Nδ-hydroxylation of l-ornithine in many Pseudomonas spp., and thus provides an essential enzymic function in the biogenesis of the pyoverdine siderophore. Here, we report a detailed analysis of the membrane topology of the PvdA enzyme from the bacterial pathogen Pseudomonas aeruginosa. Membrane topogenic determinants of PvdA were identified by computational analysis, and verified in Escherichia coli by constructing a series of translational fusions between PvdA and the PhoA (alkaline phosphatase) reporter enzyme. The inferred topological model resembled a eukaryotic reverse signal-anchor (type III) protein, with a single N-terminal domain anchored to the inner membrane, and the bulk of the protein spanning the cytosol. According to this model, the predicted transmembrane region should overlap the putative FAD-binding site. Cell fractionation and proteinase K accessibility experiments in P. aeruginosa confirmed the membrane-bound nature of PvdA, but excluded the transmembrane topology of its N-terminal hydrophobic region. Mutational analysis of PvdA, and complementation assays in a P. aeruginosa ΔpvdA mutant, demonstrated the dual (structural and functional) role of the PvdA N-terminal domain.
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