Structure and functional studies of the short consensus repeats of the human complement receptor type 1

Abstract

Complement Receptor Type 1 (CR1) is an integral membrane protein that consists of 30 short consensus repeats (SCR). Its binds two activated components of the complement cascade, C3b and C4b, and by binding these proteins CR1 plays an important role in the regulation of the complement cascade.

A number of constructs coding for the SCRs of the N-terminal region of CR1 were constructed, and the recombinant proteins were refolded because they were expressed as inclusion bodies in Escherichia coli.

The refolded recombinant proteins showed functional activity in both the classical and alternative pathways by inhibiting the complement-mediated lysis of red blood cells. SCR 123 and SCR 12DNET3cys were significantly more effective at inhibiting complement-mediated lysis of the classical pathway than SCR 23, SCR 23cys, SCR 122 and SCR 2DNET3cys. In the alternative pathway, the IH₅₀ value for SCR 123 was not significantly different compared to SCR12, SCR 23cys, SCR 122. However, the substitution of five amino acids from SCR 9 of LHR-B into SCR 12DNET3cys resulted in a 27-fold potency improvement in the alternative pathway compared to SCR 123. The same substitution of amino acids into SCR 2DNET3cys also significantly improved its potency compared to SCR 23.

Soluble SCR domains expressed in a bacterial system are stable and retain some of the inhibitory function native CR1. Further study of these SCR domain constructs could lead to the development of a therapeutic agent that can inhibit detrimental complement activation.

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