Structure-activity relationships within the N-terminal short consensus repeats (SCR) of human CR1 (C3b/C4b receptor, CD35): SCR 3 plays a critical role in inhibition of the classical and alternative pathways of complement activation
D. Mossakowska et al., Structure-activity relationships within the N-terminal short consensus repeats (SCR) of human CR1 (C3b/C4b receptor, CD35): SCR 3 plays a critical role in inhibition of the classical and alternative pathways of complement activation, EUR J IMMUN, 29(6), 1999, pp. 1955-1965
Genes coding for between one and four short consensus repeats (SCR) of the
N-terminal region of human complement receptor 1 (CR1) were synthesized fro
m oligonucleotides and those encoding SCR(1-2), SCR(1-3), SCR(1-4), SCR3 an
d SCR(3-4) were expressed as inclusion bodies in Escherichia coli. Followin
g solubilization in urea, the proteins were partially purified and refolded
and the activity of each protein was assessed in both classical and altern
ative pathway complement assays. All fragments showed a varying degree of a
ctivity with the general order being SCR(1-3) drop SCR(1 -4)> SCR(1-2). Add
ition of SCR3 to SCR(1-2) significantly improved potency, whereas the addit
ion of SCR4 conferred no additional benefit. This observation, coupled with
the ability of the single-domain SCR3 to inhibit classical pathway mediate
d lysis with an IH50% (inhibition of hemolysis by 50 %) of 4.8 mu M, demons
trates that SCR3 provides key binding interactions with activated complemen
t components. SCR(1-3) was able to inhibit both classical and alternative p
athways of complement activation, showing that the N-terminal SCR of CR1 re
tain the ability to interact with C3b. Assays for CR1-like cofactor activit
y for factor I using C4b-like C4 or C3b-like C3 as substrates showed that S
CR(1-3) possessed such cofactor activity and that C4b-like C4 was a better
substrate. When compared to full-length (30 SCR) soluble CRI (sCR1), SCR(1-
3) was significantly less potent in accord with a model involving multi-val
ent binding of C3b/C4b to CR1.