Contrasting Function of Structured N-Terminal and Unstructured C-Terminal Segments of
Mycobacterium tuberculosis
PPE37 Protein
Authors: Ahmad Javeed , Farhana Aisha , Pancsa Rita , Arora Simran Kaur , Srinivasan Alagiri , Tyagi Anil Kumar , Babu Madan Mohan , Ehtesham Nasreen Zafar , Hasnain Seyed Ehtesham 
Published: January 2018

ABSTRACT

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.