The Electrostatic Nature of C3d-Complement Receptor 2 Association

Abstract

The association of complement component C3d with B or T cell complement receptor 2 (CR2 or CD21) is a link between innate and adaptive immunity. It has been recognized in experimental studies that the C3d-CR2 association is pH- and ionic strength-dependent. This led us to perform electrostatic calculations to obtain a theoretical understanding of the mechanism of C3d-CR2 association. We used the crystallographic structures of human free C3d, free CR2 (short consensus repeat (SCR)1–2), and the C3d-CR2(SCR1–2) complex, and continuum solvent representation, to obtain a detailed atomic-level picture of the components of the two molecules that contribute to association. Based on the calculation of electrostatic potentials for the free and bound species and apparent pKa values for each ionizable residue, we show that C3d-CR2(SCR1–2) recognition is electrostatic in nature and involves not only the association interface, but also the whole molecules. Our results are in qualitative agreement with experimental data that measured the ionic strength and pH dependence of C3d-CR2 association. Also, our results for the native molecules and a number of theoretical mutants of C3d explain experimental mutagenesis studies of amino acid replacements away from the association interface that modulate binding of iC3b with full-length CR2. Finally, we discuss the packing of the two SCR domains. Overall, our data provide global and site-specific explanations of the physical causes that underlie the ionic strength dependence of C3d-CR2 association in a unified model that accounts for all experimental data, some of which were previously thought to be contradictory.

Mapping of the C3d Ligand Binding Site on Complement Receptor 2 (CR2/CD21) Using Nuclear Magnetic Resonance and Chemical Shift Analysis

“…From these data, an IC 50 was calculated and listed in Table 1. The IC 50 for the C3d peptide (C3dp1) was 100 Ϯ 8 M, and for the control peptide was greater than 500 M. The peptide C3dp1 was found to demonstrate dose-dependent inhibition (Fig. 2).…”

“…The CR2-C3d interaction has been studied, and the results of site-directed mutagenesis suggest that both SCR domains are required for binding and that the interaction is in part charge-mediated (36). Residues found to be required for interaction were Arg 13 , Tyr 16 , Ser 32 , Arg 28 , Arg 36 , Lys 41 , Lys 50 , Lys 57 , Lys 67 , and Arg 83 . Despite substantial progress in understanding CR2-ligand interactions, several key questions remain.…”

Structural insights into the central complement component C3

“…This provides a link between the innate and the adaptive immune system (Carroll, 2004). Multiple studies have been performed on the interaction of CR2 with iC3b and C3dg (Clemenza and Isenman, 2000;Diefenbach and Isenman, 1995;Esparza et al, 1991;Kalli et al, 1991;Lambris et al, 1985;Lowell et al, 1989;Molina et al, 1995;Morikis and Lambris, 2004;Sarrias et al, 2001) (Fig. 2).…”

Characterization of Human Complement Receptor Type 2 (CR2/CD21) as a Receptor for IFN-α: A Potential Role in Systemic Lupus Erythematosus

“…Substantial insights into the structure and biophysical characteristics of the ligand recognition mechanism of CR2 were provided by the solution of the x-ray crystallographic structure of the short consensus repeat (SCR)1-2 domain of CR2 with C3d (6) as well as without the C3d ligand (7). Subsequent in silico modeling (8), analytical ultracentrifugation, x-ray and neutron scattering (9), as well as mutagenesis (10) analyses have resulted in the current understanding that the SCR1-2 domain of CR2 interacts with C3d by both charge-dependent and charge-independent means. CR2 has been primarily viewed as a B cell coreceptor whose biologic role is to amplify Ag receptor-mediated signal transduction.…”