Burmeister R, Boe I M, Nykjaer A, Jacobsen C, Moestrup S K, Verroust P, Christensen E I, Lund J, Willnow T E
Max-Delbrueck-Center for Molecular Medicine, 13125 Berlin, Germany.
J Biol Chem. 2001 Apr 20;276(16):13295-301. doi: 10.1074/jbc.M010679200. Epub 2001 Jan 22.
Clara cell secretory protein (CCSP) is a transport protein for lipophilic substances in bronchio-alveolar fluid, plasma, and uterine secretion. It acts as a carrier for steroid hormones and polychlorinated biphenyl metabolites. Previously, the existence of receptors for uptake of CCSP.ligand complexes into the renal proximal tubules had been suggested. Using surface plasmon resonance analysis, we demonstrate that CCSP binds to cubilin, a peripheral membrane protein on the surface of proximal tubular cells. Binding to cubilin results in uptake and lysosomal degradation of CCSP in cultured cells. Surprisingly, internalization of CCSP is blocked not only by cubilin antagonists but also by antibodies directed against megalin, an endocytic receptor that does not bind CCSP but associates with cubilin. Consistent with a role of both receptors in renal uptake of CCSP in vivo, patients deficient for cubilin or mice lacking megalin exhibit a defect in tubular uptake of the protein and excrete CCSP into the urine. These findings identify a cellular pathway consisting of a CCSP-binding protein (cubilin) and an endocytic coreceptor (megalin) responsible for tissue-specific uptake of CCSP and associated ligands.
克拉拉细胞分泌蛋白（CCSP）是支气管肺泡液、血浆和子宫分泌物中亲脂性物质的转运蛋白。它作为类固醇激素和多氯联苯代谢产物的载体。此前，有人提出肾近端小管存在摄取CCSP-配体复合物的受体。利用表面等离子体共振分析，我们证明CCSP与cubilin结合，cubilin是近端小管细胞表面的一种外周膜蛋白。与cubilin结合导致CCSP在培养细胞中摄取并被溶酶体降解。令人惊讶的是，CCSP的内化不仅被cubilin拮抗剂阻断，还被针对巨膜蛋白的抗体阻断，巨膜蛋白是一种不与CCSP结合但与cubilin相关的内吞受体。与这两种受体在体内肾摄取CCSP中的作用一致，缺乏cubilin的患者或缺乏巨膜蛋白的小鼠表现出该蛋白在肾小管摄取方面的缺陷，并将CCSP排泄到尿液中。这些发现确定了一条由CCSP结合蛋白（cubilin）和内吞共受体（巨膜蛋白）组成的细胞途径，该途径负责CCSP及相关配体的组织特异性摄取。
