Proteins with bacterial
immunoglobulin-like (Big) domains, such as the Yersinia pseudotuberculosis invasin and Escherichia coli intimin, are surface-expressed
proteins that mediate host mammalian cell invasion or attachment. Here, we report the identification and characterization of a new family of Big domain
proteins, referred to as Lig (leptospiral Ig-like)
proteins, in pathogenic Leptospira. Screening of L. interrogans and L. kirschneri expression libraries with sera from
leptospirosis patients identified 13 lambda phage clones that encode tandem repeats of the 90
amino acid Big domain. Two lig genes, designated ligA and ligB, and one pseudogene, ligC, were identified. The ligA and ligB genes encode amino-terminal
lipoprotein signal peptides followed by 10 or 11 Big domain repeats and, in the case of ligB, a unique carboxy-terminal non-repeat domain. The organization of ligC is similar to that of ligB but contains mutations that disrupt the reading frame. The lig sequences are present in pathogenic but not saprophytic Leptospira species. LigA and LigB are expressed by a variety of virulent leptospiral strains. Loss of Lig
protein and
RNA transcript expression is correlated with the observed loss of virulence during culture attenuation of pathogenic strains. High-pressure freeze substitution followed by immunocytochemical electron microscopy confirmed that the Lig
proteins were localized to the bacterial surface. Immunoblot studies with patient sera found that the Lig
proteins are a major
antigen recognized during the acute host
infection. These observations demonstrate that the Lig
proteins are a newly identified
surface protein of pathogenic Leptospira, which by analogy to other bacterial
immunoglobulin superfamily
virulence factors, may play a role in host cell attachment and invasion during leptospiral pathogenesis.