Structural insights into the dual strategy of recognition by peptidoglycan recognition protein, PGRP-S: structure of the ternary complex of PGRP-S with lipopolysaccharide and stearic acid

PLoS One. 2013;8(1):e53756. doi: 10.1371/journal.pone.0053756. Epub 2013 Jan 9.

Abstract

Peptidoglycan recognition proteins (PGRPs) are part of the innate immune system. The 19 kDa Short PGRP (PGRP-S) is one of the four mammalian PGRPs. The concentration of PGRP-S in camel (CPGRP-S) has been shown to increase considerably during mastitis. The structure of CPGRP-S consists of four protein molecules designated as A, B, C and D forming stable intermolecular contacts, A-B and C-D. The A-B and C-D interfaces are located on the opposite sides of the same monomer leading to the the formation of a linear chain with alternating A-B and C-D contacts. Two ligand binding sites, one at C-D contact and another at A-B contact have been observed. CPGRP-S binds to the components of bacterial cell wall molecules such as lipopolysaccharide (LPS), lipoteichoic acid (LTA), and peptidoglycan (PGN) from both gram-positive and gram-negative bacteria. It also binds to fatty acids including mycolic acid of the Mycobacterium tuberculosis (Mtb). Previous structural studies of binary complexes of CPGRP-S with LPS and stearic acid (SA) have shown that LPS binds to CPGRP-S at C-D contact (Site-1) while SA binds to it at the A-B contact (Site-2). The binding studies using surface plasmon resonance showed that LPS and SA bound to CPGRP-S in the presence of each other. The structure determination of the ternary complex showed that LPS and SA bound to CPGRP-S at Site-1 and Site-2 respectively. LPS formed 13 hydrogen bonds and 159 van der Waals contacts (distances ≤4.2 Å) while SA formed 56 van der Waals contacts. The ELISA test showed that increased levels of productions of pro-inflammatory cytokines TNF-α and IFN-γ due to LPS and SA decreased considerably upon the addition of CPGRP-S.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Binding Sites
  • Camelus
  • Carrier Proteins / chemistry*
  • Carrier Proteins / metabolism
  • Crystallography, X-Ray
  • Humans
  • Hydrogen Bonding
  • Ligands
  • Lipopolysaccharides / chemistry*
  • Mycobacterium tuberculosis / chemistry
  • Mycolic Acids / chemistry
  • Protein Binding
  • Protein Conformation
  • Protein Structure, Tertiary
  • Stearic Acids / chemistry*
  • Ternary Complex Factors / chemistry*

Substances

  • Carrier Proteins
  • Ligands
  • Lipopolysaccharides
  • Mycolic Acids
  • Stearic Acids
  • Ternary Complex Factors
  • peptidoglycan recognition protein
  • stearic acid

Grants and funding

The authors acknowledge financial support from the Department of Science and Technology (DST), Ministry of Science and Technology, New Delhi. TPS thanks the Department of Biotechnology (DBT), Ministry of Science and Technology, New Delhi for the award of Distinguished Biotechnology research professorship to him. PS thanks the DST for the INSPIRE Faculty award to him. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.