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Journal of Bacteriology, September 2005, p. 6528-6535, Vol. 187, No. 18
0021-9193/05/$08.00+0     doi:10.1128/JB.187.18.6528-6535.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Characterization of a Novel Porin Protein from Moraxella catarrhalis and Identification of an Immunodominant Surface Loop

Donna M. Easton,1 Adam Smith,1 Sara Gomez Gallego,1 A. Ruth Foxwell,1 Allan W. Cripps,1,2 and Jennelle M. Kyd1*

Division of Health, Design and Science, Gadi Research Centre, University of Canberra, Canberra, Australia,1 School of Medicine, Griffith University, Gold Coast Campus, Gold Coast, Australia2

Received 3 February 2005/ Accepted 15 June 2005

Moraxella catarrhalis is a gram-negative bacterium that is mainly responsible for respiratory tract infections. In this study we report a novel outer membrane protein (OMP), designated M35, with a molecular mass of 36.1 kDa. This protein was structurally homologous to classic gram-negative porins, such as OMP C from Escherichia coli and OMP K36 from Klebsiella pneumoniae, with a predicted structure of 8 surface loops and 16 antiparallel ß-sheets. The DNA sequences of the genes from 18 diverse clinical isolates showed that the gene was highly conserved (99.6 to 100% of nucleotides), with only one isolate (ID78LN266) having base variations that resulted in amino acid substitutions. Electrophoresis and analysis of recognition of the protein using mouse anti-M35 sera showed that M35 was expressed on the bacterial surface and constitutively expressed across M. catarrhalis isolates, with only ID78LN266 showing poor antibody recognition. Our results showed that the single amino acid mutation in loop 3 significantly affected antibody recognition, indicating that loop 3 appeared to contain an immunodominant B-cell epitope. The antibody specificity to loop 3 may be a potential mechanism for evasion of host immune responses targeted to M35, since loop 3 should theoretically orientate into the porin channel. Thus, M35 is a highly conserved, surface-expressed protein that is of significance for its potential functional role as an M. catarrhalis porin and is of interest as a vaccine candidate.

* Corresponding author. Mailing address: Division of Health, Design and Science, University of Canberra, Canberra, ACT 2601, Australia. Phone: 61-2-6201 2160. Fax: 61-2-6201 2461. E-mail: jennelle.kyd{at}canberra.edu.au.

Journal of Bacteriology, September 2005, p. 6528-6535, Vol. 187, No. 18
0021-9193/05/$08.00+0     doi:10.1128/JB.187.18.6528-6535.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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