Molecular cloning and characterization of outer membrane protein E of Moraxella (Branhamella) catarrhalis

Outer membrane protein E (OMP E) is a 50-kDa protein of Moraxella (Branhamella) catarrhalis. It is a potential vaccine antigen because it is expressed on the surface of the bacterium and has antigenic determinants which are conserved among most strains of M. catarrhalis. To clone the gene encoding OMP E, an EMBL-3 genomic library of strain 25240 was screened with a family of degenerate oligonucleotides based on the amino-terminal protein sequence. The OMP E gene was identified in one of the six positive clones by Southern blot analysis. An open reading frame of 1,377 bp encoding a protein of 460 amino acids was identified. The calculated molecular mass of the mature protein of 436 amino acid residues was 47.03 kDa, which correlated well with the results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein product of the OMP E gene had a leader peptide of 25 amino acids and a signal peptidase 1 cleavage site similar to those of known OMPs of Escherichia coli. The transcription initiation site of the OMP E gene was mapped by primer extension to be 78 nucleotides upstream of the ATG start codon. Borderline homology was found to the FadL protein of E. coli (49.1% similarity and 25.6% identity), which is involved in the binding and transport of fatty acids. Analysis of restriction fragment length polymorphisms of the OMP E genes of 19 different strains of M. catarrhalis showed that the OMP E gene is highly conserved. The high degree of conservation of sequences of the OMP E genes of M. catarrhalis from diverse sources, along with earlier observations that the protein contains antigenic determinants on the bacterial surface, indicates that OMP E should be studied further as a potential vaccine antigen.