Complement factor H (FH), an elongated and substantially glycosylated 20-domain protein, is a soluble regulator of the complement alternative pathway (AP). It contains several glycan binding sites which mediate recognition of α2-3-linked sialic acid (FH domain 20) and glycosaminoglycans (domains 6–8 and 19–20). FH also binds the complement C3-activation product C3b, a powerful opsonin and focal point for the formation of C3-convertases of the AP feedback loop. In freely circulating FH the C3b binding site in domains 19–20 is occluded, a phenomenon that is not fully understood and could be mediated by an intramolecular interaction between FH’s intrinsic sialylated glycosylation and its own sialic acid binding site. In order to assess this possibility, we characterized FH’s sialylation with respect to glycosidic linkage type and searched for further potential, not yet characterized sialic acid binding sites in FH and its seven-domain spanning splice variant and fellow complement regulator FH like-1 (FHL-1). We also probed FH binding to the sialic acid variant Neu5Gc which is not expressed in humans but on heterologous erythrocytes that restrict the human AP and in FH transgenic mice. We find that FH contains mostly α2-6-linked sialic acid, making an intramolecular interaction with its α2-3-sialic acid specific binding site and an associated self-lock mechanism unlikely, substantiate that there is only a single sialic acid binding site in FH and none in FHL-1, and demonstrate direct binding of FH to the nonhuman sialic acid Neu5Gc, supporting the use of FH transgenic mouse models for studies of complement-related diseases.

中文翻译：

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补体调节因子H对唾液酸识别的生物物理分析

补体因子H（FH）是一种伸长的且基本上糖基化的20结构域蛋白，是补体替代途径（AP）的可溶性调节剂。它包含几个聚糖结合位点，介导α2-3连接的唾液酸（FH结构域20）和糖胺聚糖（结构域6-8和19-20）的识别。FH还与补体C3激活产物C3b结合，C3b激活产物C3b是强大的调理素，是AP反馈回路C3转化酶形成的焦点。在自由循环的FH中，域19-20中的C3b结合位点被封闭，这种现象尚未完全理解，可能是由FH内在的唾液酸化糖基化与其自身的唾液酸结合位点之间的分子内相互作用介导的。为了评估这种可能性，我们针对糖苷键类型对FH的唾液酸化进行了表征，并在FH及其七结构域剪接变体和互补补体调节因子FH like-1（FHL-1）中寻找了进一步的潜力，尚未表征唾液酸的结合位点。我们还探讨了FH与唾液酸变体Neu5Gc的结合，后者在人类中未表达，但在限制人类AP的异源红细胞和FH转基因小鼠中表达。我们发现FH主要包含α2-6-连接的唾液酸，使其与其α2-3-唾液酸特异性结合位点和相关的自锁机制的分子内相互作用不太可能发生，这证明在FH中仅存在一个唾液酸结合位点。 FH在FHL-1中没有，并且证明FH与非人类唾液酸Neu5Gc直接结合，