Poon P H, Schumaker V N
Department of Chemistry and Biochemistry, University of California, Los Angeles 90024.
J Biol Chem. 1991 Mar 25;266(9):5723-7.
The interactions between the complement components and with immunoglobulins are greatly enhanced by lowering the ionic strength and become readily measurable by physical techniques. Thus, the binding between C1q and IgM was previously shown to be appreciable (k = 1 x 10(6) M-1) at 0.084 M ionic strength (Poon, P.H., Phillips, M.L., and Schumaker, V.N. (1985) J. Biol. Chem. 260, 9357-9365). We have now found that, at 0.128 M ionic strength, the binding between human C1- (the activated first component of complement) and IgM was strong at physiological concentrations (k = 1 x 10(7) M-1), while under the same conditions binding between C1q and IgM was not observed. To explore the nature of the interactions responsible for this enhanced binding by C1- over C1q, mixtures of the various subcomponents of C1- were studied alone and with IgM. C1r2 did not bind to C1q, even when the ionic strength was reduced to 0.098 M, nor did the presence of C1r2 enhance the binding of C1q to IgM. In contrast, two C1s2 independently bound to C1q (k = 1 x 10(6) M-1), and caused a marked increase in its association with IgM (k = 5 x 10(6) M-1) at 0.098 M ionic strength. No detectable interaction was found between C1s2 and/or C1r2 and IgM in the absence of C1q. Moreover, there was no detectable interaction between the C1(-)-like complex formed between C1r2C1s2 and the collagenous C1q stalks (pepsin-digested C1q) and IgM. These data suggest that the binding of C1s2 to C1q, either alone or together with C1r2, induces a conformational change in C1q which results in additional C1q heads binding to complementary sites on IgM.
通过降低离子强度，补体成分之间以及与免疫球蛋白之间的相互作用会大大增强，并且可以通过物理技术轻松测量。因此，先前已表明，在离子强度为0.084 M时，C1q与IgM之间的结合相当可观（k = 1×10⁶ M⁻¹）（Poon，P.H.，Phillips，M.L.，和Schumaker，V.N.（1985年）《生物化学杂志》260，9357 - 9365）。我们现在发现，在离子强度为0.128 M时，人C1 - （补体的活化第一成分）与IgM在生理浓度下的结合很强（k = 1×10⁷ M⁻¹），而在相同条件下未观察到C1q与IgM之间的结合。为了探究导致C1 - 比C1q结合增强的相互作用的本质，研究了C1 - 的各种亚成分单独以及与IgM混合的情况。即使离子强度降至0.098 M，C1r₂也不与C1q结合，C1r₂的存在也不会增强C1q与IgM的结合。相比之下，两个C1s₂独立地与C1q结合（k = 1×10⁶ M⁻¹），并在离子强度为0.098 M时使其与IgM的结合显著增加（k = 5×10⁶ M⁻¹）。在没有C1q的情况下，未发现C1s₂和/或C1r₂与IgM之间有可检测到的相互作用。此外，在C1r₂C1s₂与胶原质C1q茎（胃蛋白酶消化的C1q）形成的类似C1（ - ）的复合物与IgM之间未发现可检测到的相互作用。这些数据表明，C1s₂单独或与C1r₂一起与C1q的结合会诱导C1q的构象变化，从而导致额外的C1q头部与IgM上的互补位点结合。
