Zlatarova Alexandra S, Rouseva Marieta, Roumenina Lubka T, Gadjeva Mihaela, Kolev Martin, Dobrev Ivan, Olova Neli, Ghai Rohit, Jensenius Jens Chr, Reid Kenneth B M, Kishore Uday, Kojouharova Mihaela S
Department of Biochemistry, Sofia University, St. Kliment Ohridski, 8 Dragan Tzankov Strasse, Sofia 1164, Bulgaria.
Biochemistry. 2006 Aug 22;45(33):9979-88. doi: 10.1021/bi060539v.
C1q is the first subcomponent of the classical complement pathway that binds antigen-bound IgG or IgM and initiates complement activation via association of serine proteases C1r and C1s. The globular domain of C1q (gC1q), which is the ligand-recognition domain, is a heterotrimeric structure composed of the C-terminal regions of A (ghA), B (ghB), and C (ghC) chains. The expression and functional characterization of ghA, ghB, and ghC modules have revealed that each chain has some structural and functional autonomy. Although a number of studies have tried to identify IgG-binding sites on the gC1q domain, no such attempt has been made to localize IgM-binding site. On the basis of the information available via the gC1q crystal structure, molecular modeling, mutational studies, and bioinformatics, we have generated a series of substitution mutants of ghA, ghB, and ghC and examined their interactions with IgM. The comparative analysis of IgM- and IgG-binding abilities of the mutants suggests that the IgG- and IgM-binding sites within the gC1q domain are different but may overlap. Whereas Arg(B108), Arg (B109), and Tyr(B175) mainly constitute the IgM-binding site, the residues Arg(B114), Arg(B129), Arg(B163), and His(B117) that have been shown to be central to IgG binding are not important for the C1q-IgM interaction. Given the location of Arg(B108), Arg (B109), and Tyr(B175) in the gC1q crystal structure, it is likely that C1q interacts with IgM via the top of the gC1q domain.
C1q是经典补体途径的首个亚组分，它能结合抗原结合的IgG或IgM，并通过丝氨酸蛋白酶C1r和C1s的结合启动补体激活。C1q的球状结构域（gC1q）是配体识别结构域，是一种由A链（ghA）、B链（ghB）和C链（ghC）的C末端区域组成的异源三聚体结构。ghA、ghB和ghC模块的表达及功能特性表明，每条链都具有一定的结构和功能自主性。尽管许多研究试图确定gC1q结构域上的IgG结合位点，但尚未有人尝试定位IgM结合位点。基于通过gC1q晶体结构、分子建模、突变研究和生物信息学获得的信息，我们构建了一系列ghA、ghB和ghC的替代突变体，并检测了它们与IgM的相互作用。对突变体的IgM和IgG结合能力的比较分析表明，gC1q结构域内的IgG和IgM结合位点不同，但可能存在重叠。虽然Arg（B108）、Arg（B109）和Tyr（B175）主要构成IgM结合位点，但已证明对IgG结合至关重要的残基Arg（B114）、Arg（B129）、Arg（B163）和His（B117）对C1q-IgM相互作用并不重要。鉴于Arg（B108）、Arg（B109）和Tyr（B175）在gC1q晶体结构中的位置，C1q可能通过gC1q结构域的顶部与IgM相互作用。
