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Analysis of the Interaction between Globular Head Modules of Human C1q and Its Candidate Receptor gC1qR

Lina Pednekar¹, Ansar A Pathan¹, Basudev Paudyal¹

¹Biosciences, College of Health and Life Sciences, Brunel University London , London , UK.

Frontiers in Immunology

|December 27, 2016

Insights

This study identifies key binding sites between the globular head (gC1q) of human C1q and its receptor gC1qR. These findings illuminate C1q-gC1qR interactions on immune cells, impacting inflammation and immunity.

Area of Science:

Immunology
Molecular Biology
Biochemistry

Background:

The globular head (gC1q) of human C1q interacts with the pattern recognition protein gC1qR.
Understanding this interaction is crucial for elucidating gC1qR's diverse functions.

Purpose of the Study:

To map the gC1qR-binding site on the gC1q domain of human C1q.
To investigate the functional implications of C1q-gC1qR interactions in immune responses.

Main Methods:

Utilized recombinant ghA, ghB, and ghC modules and substitution mutants of human C1q.
Employed quantitative PCR (qPCR) to analyze gene expression.
Conducted microscopic studies and cell proliferation assays.

Main Results:

Identified specific residues (Arg162 in ghA, residue 114 in ghB) critical for C1q-gC1qR binding.
Demonstrated independent binding of ghA, ghB, and ghC modules to gC1qR.
Showed C1q and gC1qR colocalization on PBMCs and their roles in modulating immune cell proliferation.

Conclusions:

Pinpointed key residues mediating C1q-gC1qR interactions.
Established an autocrine/paracrine basis for C1q-gC1qR interaction on monocytes.
Highlighted the relevance of these interactions for C1q-mediated functions in inflammation, infection, and immunity.

Read full analysis

Abstract:

The heterotrimeric globular head (gC1q) domain of human C1q is made up of the C-terminal ends of the three individual chains, ghA, ghB, and ghC. A candidate receptor for the gC1q domain is a multi-functional pattern recognition protein, gC1qR. Since understanding of gC1qR and gC1q interaction could provide an insight into the pleiotropic functions of gC1qR, this study was undertaken to identify the gC1qR-binding site on the gC1q domain, using the recombinant ghA, ghB, and ghC modules and their substitution mutants. Our results show that ghA, ghB, and ghC modules can interact with gC1qR independently, thus reinforcing the notion of modularity within the gC1q domain of human C1q. Mutational analysis revealed that while Arg162 in the ghA module is central to interaction between gC1qR and C1q, a single amino acid substitution (arginine to glutamate) in residue 114 of the ghB module resulted in enhanced binding. Expression of gC1qR and C1q in adherent monocytes with or without pro-inflammatory stimuli was also analyzed by qPCR; it showed an autocrine/paracrine basis of C1q and gC1qR interaction. Microscopic studies revealed that C1q and gC1qR are colocalized on PBMCs. Cell proliferation assays indicated that ghA, ghB, and ghC modules were able to attenuate phytohemagglutinin-stimulated proliferation of PBMCs. Addition of gC1qR had an additive effect on the anti-proliferative effect of globular head modules. In summary, our results identify residues involved in C1q-gC1qR interaction and explain, to a certain level, their involvement on the immune cell surface, which is relevant for C1q-induced functions including inflammation, infection, and immunity.

Keywords:

C1q cell proliferation gC1qR globular head protein–protein interaction

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