Mycobacterium smegmatis laminin‐binding glycoprotein shares epitopes with Mycobacterium tuberculosis heparin‐binding haemagglutinin

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Abstract
Mycobacterium tuberculosis, the causative agent of tuberculosis, produces a heparin‐binding haemagglutinin adhesin (HBHA), which is involved in its epithelial adherence. To ascertain whether HBHA is also present in fast‐growing mycobacteria, Mycobacterium smegmatis was studied using anti‐HBHA monoclonal antibodies (mAbs). A cross‐reactive protein was detected by immunoblotting of M. smegmatis whole‐cell lysates. However, the M. tuberculosis HBHA‐encoding gene failed to hybridize with M. smegmatis chromosomal DNA in Southern blot analyses. The M. smegmatis protein recognized by the anti‐HBHA mAbs was purified by heparin–Sepharose chromatography, and its amino‐terminal sequence was found to be identical to that of the previously described histone‐like protein, indicating that M. smegmatis does not produce HBHA. Biochemical analysis of the M. smegmatis histone‐like protein shows that it is glycosylated like HBHA. Immunoelectron microscopy demonstrated that the M. smegmatis protein is present on the mycobacterial surface, a cellular localization inconsistent with a histone‐like function, but compatible with an adhesin activity. In vitro protein interaction assays showed that this glycoprotein binds to laminin, a major component of basement membranes. Therefore, the protein was called M. smegmatis laminin‐binding protein (MS‐LBP). MS‐LBP does not appear to be involved in adherence in the absence of laminin but is responsible for the laminin‐mediated mycobacterial adherence to human pneumocytes and macrophages. Homologous laminin‐binding adhesins are also produced by virulent mycobacteria such as M. tuberculosis and Mycobacterium leprae, suggesting that this adherence mechanism may contribute to the pathogenesis of mycobacterial diseases.