Immunophysical Evaluation of the Initiating Step in the Formation of the Membrane Attack Complex

Abstract

The complex between complement system proteins C5b and C6 is the cornerstone for the assembly of the membrane attack complex (MAC, also known as C5b6789 n). MAC is the terminal product of three converging pathways of the complement system and functions as a pore forming complex on cell surfaces, as a response of the immune system in fighting pathogens. However, when proper regulation of the complement system is compromised, MAC also attacks host tissues and contributes to several complement-mediated autoimmune diseases. We performed a molecular dynamics and electrostatics study to elucidate the mechanism of interaction between C5b and C6 and the formation of the C5b6 complex. The C5b6 interface consists of three binding sites stabilized predominantly by van der Waals interactions, and several critical salt bridges and hydrogen bonds. We discuss differences between domains C5d and C3d that lead to mono-functionality of C5d in acting as the scaffold for MAC formation, as opposed to dual functionality of C3d in acting as an opsonin for phagocytosis and as a link between innate and adaptive immunity, based on a comparative sequence, structural, and physicochemical analysis. We also extended our analysis to pathway dynamics to demonstrate the significance of consumption-production rates of C5b, C6, and C5b6 that lead toward MAC formation. Finally, we propose that C5d is a target for drug discovery, aiming to the inhibition of the MAC formation in autoimmune diseases originating from MAC-mediated host cell lysis.

Role of Electrostatic Hotspots in the Selectivity of Complement Control Proteins Toward Human and Bovine Complement Inhibition

“…Electrostatic hotspot analysis has been performed using the AESOP (Analysis of Electrostatic Similarities Of Proteins) code, developed by Morikis ( Kieslich et al, 2011b ; Harrison et al, 2017 ) and extensively employed for electrostatic analysis of proteins ( Kieslich et al, 2011a , Kieslich et al, 2011b ; Gorham et al, 2011a , Gorham et al, 2011b ; Harrison et al, 2015 ; Mohan et al, 2015 , Mohan et al, 2016 ; Zewde et al, 2018 ; Harrison and Morikis, 2019 ). AESOP enables the analysis of electrostatic hotspots through the calculation of an Electrostatic Similarity Indices (ESI), ( Kieslich and Morikis, 2012 ), which assesses the electrostatic potential upon perturbation (i.e., mutagenesis), identifying regions of high electrostatic similarity, or those regions least affected by perturbation.…”

Mutational signatures in GATA3 transcription factor and its DNA binding domain that stimulate breast cancer and HDR syndrome

“…Similar to FH , mutations in the MAC complex ( C5b6 ) can lead to autoimmune diseases. Correspondingly, an electrostatic study of the interaction between C5b and C6 complement proteins was completed in 38 . Electrostatic similarity methods applied to perturbed structures of C3d and Cr2 revealed electrostatic “hot-spots” at the two functional sites of C3d and a lack of electrostatic “hot-spots” at the surface of Cr2 , despite its excessively positive nature 39 .…”

The role of the complement system in disc degeneration and Modic changes

“…This indicates involvement of the classical and alternative complement pathway and formation of TCC complex. 75 Furthermore, the proteoglycans and complement interactors fibromodulin (FMOD), BGN, and decorin (DCN), which are all capable of binding to C1q and activating/inhibiting the classical complement system pathway, were found to be upregulated in MC BM. A further study assessed MC BM aspirates from patients undergoing spinal fusion surgery.…”