The conserved 18,000-molecular-weight outer membrane protein of Haemophilus ducreyi has homology to PAL.

Haemophilus ducreyi expresses an 18,000-molecular-weight outer membrane protein that contains a conserved surface-exposed epitope recognized by monoclonal antibody 3B9. Monoclonal antibody 3B9 cross-reacts with proteins of similar molecular weight found in many Haemophilus sp. strains, including P6, a candidate vaccine for Haemophilus influenzae. The gene encoding the 18,000-molecular-weight outer membrane protein was identified by screening a lambdagt11 genomic library with 3B9. The coding sequence of the gene was localized to a 471-bp open reading frame, designated pal (peptidoglycan-associated lipoprotein). Translation of pal predicted a mature polypeptide with a molecular weight of 15,000 that had extensive homology with P6 and Escherichia coli PAL. The predicted signal peptide had features characteristic of a prokaryotic lipoprotein, and processing of PAL was sensitive to globomycin in H. ducreyi. The sequences encoding mature H. ducreyi PAL were subcloned into the vector pRSET B and expressed as a polyhistidine-containing fusion protein that bound 3B9. In Western blot (immunoblot) analysis, serum samples obtained from healthy subjects and patients with chancroid or other genital ulcer diseases contained antibodies to purified PAL. Antibodies that bound to PAL were removed by absorption with a lysate of Haemophilus sp. antigens, suggesting that patients with chancroid do not develop an H. ducreyi-specific antibody response to PAL.