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Neisserial TonB-dependent outer-membrane proteins: detection, regulation and distribution of three putative candidates identified from the genome sequences

Departments of Medicine¹ and Microbiology and Immunology³, School of Medicine, University of North Carolina, Chapel Hill, NC 27599, USA
Department of Microbiology and Immunology, 1510 Clifton Road, Emory University, AK 30322, USA²
Meningococcal Research Group, Division of Microbiology and Infectious Diseases, University of Nottingham, University Hospital, Nottingham NG7 2UH, UK⁴

Author for correspondence: Paul C. Turner. Tel: +44 117 928 3241. Fax: +44 117 929 9162. e-mail: paul.turner{at}bristol.ac.uk

Computer searches were carried out of the gonococcal and meningococcal genome databases for previously unknown members of the TonB-dependent family (Tdf) of outer-membrane receptor proteins. Seven putative non-contiguous genes were found and three of these (identified in gonococcal strain FA1090) were chosen for further study. Consensus motif analysis of the peptide sequences was consistent with the three genes encoding TonB-dependent receptors. In view of the five previously characterized TonB-dependent proteins of pathogenic neisseriae, the putative genes were labelled tdfF, tdfG and tdfH. TdfF had homology with the siderophore receptors FpvA of Pseudomonas aeruginosa and FhuE of Escherichia coli, whereas TdfG and TdfH had homology with the haemophore receptor HasR of Serratia marcescens. The aim of this project was to characterize these proteins and determine their expression, regulation, distribution and surface exposure. Strain surveys of iron-stressed commensal and pathogenic neisseriae revealed that TdfF is unlikely to be expressed, TdfG is expressed by gonococci only and that TdfH is expressed by both meningococci and gonococci. Expression of TdfH was unaffected by iron availability. Susceptibility of TdfH to cleavage by proteases in live gonococci was consistent with surface exposure of this protein. TdfH may function as a TonB-dependent receptor for a non-iron nutrient source. Furthermore, TdfH is worthy of future investigation as a potential meningococcal vaccine candidate as it is a highly conserved, widely distributed and surface-exposed outer-membrane protein.

Keywords: gonococcus, meningococcus, TonB-dependent proteins, iron-regulated proteins

Abbreviations: HmBP, haemin-binding protein; OMP, outer-membrane protein

The GenBank accession number for the sequence of tdfH from meningococcal strain IR1074 reported in this paper is AF227418.

^a Present address: Department of Pathology and Microbiology, School of Medical Sciences, University of Bristol, University Walk, Bristol BS8 1TD, UK.

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