Sun Q H, DeLisser H M, Zukowski M M, Paddock C, Albelda S M, Newman P J
Blood Research Institute, Blood Center of Southeastern Wisconsin, Milwaukee 53233-2194, USA.
J Biol Chem. 1996 May 10;271(19):11090-8. doi: 10.1074/jbc.271.19.11090.
PECAM-1 (CD31) is a 130-kDa member of the immunoglobulin (Ig) gene superfamily that is constitutively expressed at high concentration at endothelial cell intercellular junctions and at moderate density on the surface of circulating leukocytes and platelets. Recent in vitro and in vivo studies have shown the PECAM-1 plays a central role in mediating the extravasation of leukocytes from the vessel wall in response to inflammatory mediators. To study the binding characteristics of PECAM-1, phospholipid vesicles were prepared and examined by flow cytometry and immunofluorescence microscopy for their ability to associate with each other and with cells. Proteoliposomes containing high concentrations of PECAM-1 interacted homophilically with each other, forming large self-aggregates. PECAM-1 proteoliposomes, as well as soluble bivalent PECAM-1 in the form of a PECAM-1/IgG immunoadhesin, associated homophilically with cells expressing human, but not murine, PECAM-1. This binding could be completely inhibited by monoclonal antibody Fab fragments specific for Ig homology Domain 1 or Domains 1 + 2. Binding studies using cells expressing human PECAM-1 deletion mutants and murine/human chimeras confirmed that both Ig Domains 1 and 2 were both necessary and sufficient for homophilic binding. In contrast, engagement of membrane-proximal Domain 6 with monoclonal antibody Fab fragments had the opposite effect and augmented the binding of PECAM-1 proteoliposomes to cells. Thus, PECAM-1, like certain integrins, appears to be capable of antibody-induced conformational changes that alter affinity for its ligand. Similar changes induced by physiologic stimuli could be important in regulating the function of PECAM-1 in vascular cells.
血小板内皮细胞黏附分子-1（PECAM-1，CD31）是免疫球蛋白（Ig）基因超家族中一个分子量为130 kDa的成员，在内皮细胞间连接处持续高表达，在循环白细胞和血小板表面呈中等密度表达。最近的体外和体内研究表明，PECAM-1在介导白细胞响应炎症介质从血管壁外渗过程中起核心作用。为研究PECAM-1的结合特性，制备了磷脂囊泡，并通过流式细胞术和免疫荧光显微镜检查其相互结合以及与细胞结合的能力。含有高浓度PECAM-1的蛋白脂质体彼此之间发生同源性相互作用，形成大的自聚集体。PECAM-1蛋白脂质体以及PECAM-1/IgG免疫黏附素形式的可溶性二价PECAM-1与人源而非鼠源PECAM-1表达细胞发生同源性结合。这种结合可被针对Ig同源结构域1或结构域1 + 2的单克隆抗体Fab片段完全抑制。使用表达人PECAM-1缺失突变体和鼠/人嵌合体的细胞进行的结合研究证实，Ig结构域1和2对于同源性结合都是必需且充分的。相反，用单克隆抗体Fab片段与膜近端结构域6结合则产生相反的效果，增强了PECAM-1蛋白脂质体与细胞的结合。因此，PECAM-1与某些整合素类似，似乎能够发生抗体诱导的构象变化，从而改变其对配体的亲和力。生理刺激诱导的类似变化可能在调节血管细胞中PECAM-1的功能方面具有重要意义。
