The PE_PGRS Proteins of Mycobacterium tuberculosis Are Ca(2+) Binding Mediators of Host-Pathogen Interaction

The phenomenal success of Mycobacterium tuberculosis (M.tb) as a pathogen is primarily based on its ability to modulate host immune responses. The genome of M.tb encodes multiple immunomodulatory proteins, including several members of the multigenic PE_PPE family of which the PE_PGRS proteins are a...

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Bibliographic Details
Published in:Biochemistry (Easton) 2016-08, Vol.55 (33), p.4675-4687
Main Authors: Yeruva, Veena C, Kulkarni, Apoorva, Khandelwal, Radhika, Sharma, Yogendra, Raghunand, Tirumalai R
Format: Article
Language:English
Subjects:
Bacterial Proteins - chemistry
Calcium - chemistry
Circular Dichroism
Host-Pathogen Interactions
Mycobacterium tuberculosis - chemistry
Spectrometry, Fluorescence
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Summary:The phenomenal success of Mycobacterium tuberculosis (M.tb) as a pathogen is primarily based on its ability to modulate host immune responses. The genome of M.tb encodes multiple immunomodulatory proteins, including several members of the multigenic PE_PPE family of which the PE_PGRS proteins are a subset. Curiously, 56 of the 61 PE_PGRS proteins contain multiple copies of the glycine-rich sequence motif GGXGXD/NXUX, a nonapeptide sequence predicted to bind Ca(2+), but the functional significance of these motifs remains a mystery. Here we provide evidence via isothermal titration calorimetry, (45)Ca blotting, fluorescence, and circular dichroism spectroscopy that Ca(2+) binds to the PE_PGRS proteins, PE_PGRS33 (Rv1818c) (10 motifs) and PE_PGRS61 (Rv3653) (one motif). Ca(2+) was observed not to bind to PE_PGRS8 (Rv0742), which lacks nonapeptide motifs. Using recombinant Mycobacterium smegmatis strains expressing Rv1818c and Rv3653 and the THP-1 macrophage model of infection, we show that the two proteins mediate Ca(2+)-dependent upregulation of the anti-inflammatory cytokine IL-10, events critical to the pathogenesis of M.tb. Both Rv1818c and Rv3653 interact with TLR2 in a Ca(2+)-dependent manner, providing a novel mechanistic basis for their immunomodulatory effects. Mutations in the nonapeptide motif of Rv3653 led to compromised Ca(2+) binding, validating the functional criticality of this motif. This study demonstrates for the first time not only their Ca(2+) binding properties but also an essential role for Ca(2+) in the functioning of the M.tb PE_PGRS proteins, opening up the possibility of developing novel anti-tuberculosis therapeutics that inhibit Ca(2+)-PE_PGRS binding.
ISSN:1520-4995
DOI:10.1021/acs.biochem.6b00289