Contrasting Function of Structured N-Terminal and Unstructured C-Terminal Segments of Mycobacterium tuberculosis PPE37 Protein.

Pathogens frequently employ eukaryotic linear motif (ELM)-rich intrinsically disordered proteins (IDPs) to perturb and hijack host cell networks for a productive infection. Mycobacterium tuberculosis has a relatively high percentage of IDPs in its proteome, the significance of which is not known. The Mycobacterium-specific PE-PPE protein family has several members with unusually high levels of structural disorder and disorder-promoting Ala/Gly residues. PPE37 protein, a member of this family, carries an N-terminal PPE domain capable of iron binding, two transmembrane domains, and a disordered C-terminal segment harboring ELMs and a eukaryotic nuclear localization signal (NLS). PPE37, expressed as a function of low iron stress, was cleaved by M. tuberculosis protease into N- and C-terminal segments. A recombinant N-terminal segment (P37N) caused proliferation and differentiation of monocytic THP-1 cells, into CD11c, DC-SIGN (dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin)-positive semimature dendritic cells exhibiting high interleukin-10 (IL-10) but negligible IL-12 and also low tumor necrosis factor alpha (TNF-α) secretion-an environment suitable for maintaining tolerogenic immune cells. The C-terminal segment entered the macrophage nucleus and induced caspase-3-dependent apoptosis of host cells. Mice immunized with recombinant PPE37FL and PPE37N evoked strong anti-inflammatory response, validating the in vitro immunostimulatory effect. Analysis of the IgG response of PPE37FL and PPE37N revealed significant immunoreactivities in different categories of TB patients, viz. pulmonary TB (PTB) and extrapulmonary TB (EPTB), vis-a-vis healthy controls. These results support the role of IDPs in performing contrasting activities to modulate the host processes, possibly through molecular mimicry and cross talk in two spatially distinct host environments which may likely aid M. tuberculosis survival and pathogenesis.IMPORTANCE To hijack the human host cell machinery to enable survival inside macrophages, the pathogen Mycobacterium tuberculosis requires a repertoire of proteins that can mimic host protein function and modulate host cell machinery. Here, we have shown how a single protein can play multiple functions and hijack the host cell for the benefit of the pathogen. Full-length membrane-anchored PPE37 protein is cleaved into N- and C-terminal domains under iron-depleted conditions. The N-terminal domain facilitates the propathogen semimature tolerogenic state of dendritic cells, whereas the C-terminal segment is localized into host cell nucleus and induces apoptosis. The immune implications of these in vitro observations were assessed and validated in mice and also human TB patients. This study presents novel mechanistic insight adopted by M. tuberculosis to survive inside host cells.

TlyA protein of Mycobacterium leprae: a probable bio-marker of active infection.

The extent of pathogenicity of the mycobacterial infections depends on virulence factors that mediate survival inside macrophages. Virulence factors are generally believed to be specific for pathogenic species and mutated/non-functional in nonpathogenic strains. Mycobacterial TlyA can modulate the phagolysosome maturation pathway, immediately after entry into macrophages. Over-expression of open reading frame (ORF) ML1358 (tlyA) in tissues of leprosy patients by partial DNA chip and real time PCR analysis during active infection attracted our interest to explore the properties of this gene at molecular and serological levels, to understand its role in the host. Molecular properties were studied by cloning and expression of the corresponding gene in pASK-iba 43(þ) expression vector in E. coli and bioinformatics tools while sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and ELISA were applied to investigate the serological significance of rTlyA protein in different clinical states of leprosy. We observed that TlyA has a close relation among mycobacteria with specific protein domains in slow growing intracellular adapted pathogenic species. The presence of trans-membrane domains indicates its association to the cell membrane. The study revealed its highly significant sero-reactivity (P value , 0·001) in borderline lepromatous (BL) patients, and those with reversal reaction (RR) and erythema nodosum leprosum (ENL). Its role in active infection, association with the cell membrane, presence in pathogenic species and high sero-reactivity, suggested the tlyA gene as a strong disease progression marker.