McGreal E, Gasque P
Sir William Dunn School of Pathology, University of Oxford, South Parks Road, UK.
Biochem Soc Trans. 2002 Nov;30(Pt 6):1010-4. doi: 10.1042/bst0301010.
C1q is an essential component of the phylogenetically ancient innate complement (C) system and is crucial to our natural ability to ward off infection and clear toxic cell debris (e.g. amyloid fibrils, apoptotic cells). Several candidate C1q receptors [C1q receptor for phagocytosis enhancement (C1qRp), complement receptor (CR) 1, calreticulin (CRT), binding protein for the globular head of C1q (gC1qbp)] have been described, and the aim of this review is to shed light on their structure-function relationships. One cell-surface molecule, C1qRp, has emerged as a defence collagen receptor for C1q, as well as mannose-binding lectin (MBL) and surfactant protein A. C1qRp (also known as the AA4 antigen in rodents) is the antigen recognized by a pro-adhesive monoclonal antibody called mNI-11 and antibodies against CD93, but recent results failed to confirm C1q binding activity. CR1 (CD35), a multifunctional receptor both in its ligand specificity and in its C regulation activities, is found on circulating monocytes and neutrophils, but the major site of expression is B-lymphocytes. As a receptor, CR1 binds to C1q, other C opsonins (C4b, C3b, iC3b) and MBL, and as such, has been involved in promoting phagocytosis. Several studies support a role for the cell surface receptor for the collagenous domains of C1q (cC1qR; also known as CRT). CRT belongs to the family of heat-shock proteins, the most abundant and ubiquitous soluble intracellular proteins. Though CRT does not have a transmembrane domain, it seems to mediate phagocytosis of the apoptotic cells through association with CD91. A 33 kDa protein interacts with the globular head of C1q and, logically, has been termed gC1qbp. This protein is located in mitochondria, suggesting that gC1qbp is not a cell-surface receptor itself.
C1q是系统发育上古老的固有补体（C）系统的重要组成部分，对于我们抵御感染和清除有毒细胞碎片（如淀粉样原纤维、凋亡细胞）的天然能力至关重要。已经描述了几种候选C1q受体[增强吞噬作用的C1q受体（C1qRp）、补体受体（CR）1、钙网蛋白（CRT）、C1q球形头部结合蛋白（gC1qbp）]，本综述的目的是阐明它们的结构-功能关系。一种细胞表面分子C1qRp已成为C1q、甘露糖结合凝集素（MBL）和表面活性蛋白A的防御性胶原受体。C1qRp（在啮齿动物中也称为AA4抗原）是一种称为mNI-11的促黏附单克隆抗体以及抗CD93抗体所识别的抗原，但最近的结果未能证实其C1q结合活性。CR1（CD35）在其配体特异性和补体调节活性方面都是多功能受体，存在于循环单核细胞和中性粒细胞上，但主要表达部位是B淋巴细胞。作为一种受体，CR1与C1q、其他补体调理素（C4b、C3b、iC3b）和MBL结合，因此参与促进吞噬作用。多项研究支持C1q胶原结构域的细胞表面受体[C1qR（cC1qR）；也称为CRT]发挥作用。CRT属于热休克蛋白家族，是最丰富且普遍存在的可溶性细胞内蛋白。尽管CRT没有跨膜结构域，但它似乎通过与CD91结合介导凋亡细胞的吞噬作用。一种33 kDa的蛋白与C1q的球形头部相互作用，理所当然地被称为gC1qbp。这种蛋白位于线粒体中，表明gC1qbp本身不是细胞表面受体。
