Evidence that Mycobacterial PE_PGRS Proteins Are Cell Surface Constituents That Influence Interactions with Other Cells

Abstract

The elucidation of the genomic sequence of Mycobacterium tuberculosis revealed the presence of a novel multigene family designated PE/PE_PGRS that encodes numerous, highly related proteins of unknown function. In this study, we demonstrate that a transposon insertion in a PE_PGRS gene (1818 PE_PGRS ) found in Mycobacterium bovis BCG Pasteur, which is the BCG homologue of the M. tuberculosis H37Rv gene Rv1818c, introduces new phenotypic properties to this BCG strain. These properties include dispersed growth in liquid medium and reduced infection of macrophages. Complementation of the 1818 PE_PGRS ::Tn5367 mutant with the wild-type gene restores both aggregative growth (clumping) in liquid medium and reestablishes infectivity of macrophages to levels equivalent to those for the parent BCG strain. Western blot analysis using antisera raised against the 1818 PE_PGRS protein shows that PE_PGRS proteins are found in cell lysates of BCG and M. tuberculosis H37Ra and in the cell wall fraction of M. tuberculosis H37Rv. Moreover, immunofluorescent labeling of mycobacteria indicates that certain PE_PGRS proteins are localized at the cell surface of BCG and M. tuberculosis. Together these results suggest that certain PE_PGRS proteins may be found at the surface of mycobacteria and influence both cell surface interactions among mycobacteria as well as the interactions of mycobacteria with macrophages.It is well recognized that an important first step in bacterial infection of the host is the manifestation of specific surface components by the pathogen to facilitate its entry of host cells and tissues. Many organisms have evolved novel adhesins that can bind to molecules commonly found on the eukaryotic cell surface, such as integrins (17, 38), glycosaminoglycans (16,22,41), or specific sugar residues (8, 10). A number of complex surface structures are also expressed by bacteria to seek out the appropriate host cells; these include the formation of fimbriae (pili) (2) and complex cell wall structures exemplified by the type III secretion systems (14,19,26). Following entry into the host, Mycobacterium tuberculosis shows a tropism for macrophages but can also infect epithelial cells (30,33,43). M. tuberculosis has also been shown to have ligands that bind to extracellular matrix proteins like fibronectin (1, 37, 47) and proteoglycans (16, 33). Schlesinger et al. (42) have described complement and mannose receptors on macrophages that promote the phagocytosis of mycobacteria. Genetic studies of M. tuberculosis have identified numerous genes, such as mce (3), eis (45), and erp (7), encoding proteins that enhance mycobacterial entry and survival within macrophages. Although progress has been made, the molecular mechanisms of mycobacterial infection of host cells remains unexplained.Transposon mutagenesis has been successfully used to identify novel genes that encode for bacterial virulence factors and surface components (6, 27). In the past few years, transposon mutagenesis systems specific for mycobacteria have been developed (...

Identification of outer membrane proteins of Mycobacterium tuberculosis

“…It has been proposed that the PE/PPE proteins contribute to antigenic variation of M. tuberculosis. Consistent with this function is the accumulating evidence that at least some of these proteins are accessible at the cell surface 46,84,85 . In this regard, it is striking that Rv1386 (PE15), which consists solely of a PE domain (70 amino acids without the predicted signal peptide), has high scores for both β-strand content and amphiphilicity (Table 2).…”

Association of Mycobacterium tuberculosis PE_PGRS33 polymorphism with clinical and epidemiological characteristics

“…Studies of the M. tuberculosis PE_PGRS33 gene, which encodes a surface-expressed protein, suggest that this member of the gene family has a non-redundant and uniquely important role in M. tuberculosis persistence 5,6 . Transposon mutagenesis of the Mycobacterium bovis BCG homologue of the M. tuberculosis PE_PGRS33 gene resulted in dispersed growth in liquid media and decreased ability to enter into or survive within macrophages.…”

Complex pattern of Mycobacterium marinum gene expression during long-term granulomatous infection

“…Some M. tuberculosis PE-PGRS genes are also constitutively expressed (24). Individual members of these two families play a role in Mycobacterium persistence in vivo (11,24,25). Thus, it will be especially interesting to determine whether the selective expression of certain family members in granulomas determines their specific role in bacterial persistence in vivo.…”

“…Interestingly, distinct members of the large PE͞PE-PGRS and PPE gene families were isolated both as caps, and maps or gaps (Tables 1 and 2). Some M. tuberculosis PE-PGRS genes are also constitutively expressed (24). Individual members of these two families play a role in Mycobacterium persistence in vivo (11,24,25).…”