Braulke T, Gartung C, Hasilik A, von Figura K
J Cell Biol. 1987 Jun;104(6):1735-42. doi: 10.1083/jcb.104.6.1735.
Mannose 6-phosphate-specific receptors with an apparent molecular mass of 215,000 are present in fibroblasts at the cell surface and in intracellular membranes. The cell surface receptors mediate endocytosis of exogenous lysosomal enzymes and exchange with the intracellular receptors, which function in the sorting of endogenous lysosomal enzymes. In the present study, several methods independent of receptor ligands were designed in order to examine the exchange of receptors under conditions where receptor-ligand complexes do not dissociate (weak bases and monensin) or where receptor-ligand complexes are not formed due to absence of endogenous ligands as a result of inhibition of protein synthesis. Weak bases and monensin reduce the concentration of receptors at the cell surface by 20-30% and free cell surface receptors were replaced by occupied receptors. The latter continued to be exchanged with internal ligand-occupied receptors and the rates of the exchange were similar to the control values. The exchange of receptors between the cell surface and internal membranes was also not affected when the receptor ligands were depleted from the transport compartments by treating the cells with cycloheximide for up to 10 h. We conclude from these results that movement of mannose 6-phosphate-specific receptors along the endocytosis and sorting pathways is constitutive and not triggered by binding or dissociation of ligands.
表观分子量为215,000的6-磷酸甘露糖特异性受体存在于成纤维细胞的细胞表面和细胞内膜中。细胞表面受体介导外源性溶酶体酶的内吞作用，并与细胞内受体进行交换，细胞内受体在内源性溶酶体酶的分选过程中发挥作用。在本研究中，设计了几种独立于受体配体的方法，以在受体-配体复合物不解离（弱碱和莫能菌素）或由于蛋白质合成受抑制而不存在内源性配体导致不形成受体-配体复合物的条件下，检测受体的交换情况。弱碱和莫能菌素使细胞表面受体浓度降低20%-30%，游离的细胞表面受体被占据的受体所取代。后者继续与内部被配体占据的受体进行交换，且交换速率与对照值相似。当用环己酰亚胺处理细胞长达10小时，使转运区室中的受体配体耗尽时，细胞表面与内膜之间的受体交换也不受影响。从这些结果我们得出结论，6-磷酸甘露糖特异性受体沿内吞作用和分选途径的移动是组成性的，而非由配体的结合或解离所触发。
