Apodaca G, Katz L A, Mostov K E
Department of Anatomy, University of California, San Francisco 94143.
J Cell Biol. 1994 Apr;125(1):67-86. doi: 10.1083/jcb.125.1.67.
Classically, the polymeric immunoglobulin receptor and its ligand, IgA, are thought to be sorted from basolateral early endosomes into transcytotic vesicles that directly fuse with the apical plasma membrane. In contrast, we have found that in MDCK cells IgA is delivered from basolateral endosomes to apical endosomes and only then to the apical cell surface. When internalized from the basolateral surface of MDCK cells IgA is found to accumulate under the apical plasma membrane in a compartment that is accessible to two apically added membrane markers: anti-secretory component Fab fragments, and avidin internalized from the biotinylated apical pole of the cell. This accumulation occurs in the presence of apical trypsin, which prevents internalization of the ligand from the apical cell surface. Using a modification of the diaminobenzidine density-shift assay, we estimate that approximately 80% of basolaterally internalized IgA resides in the apical endosomal compartment. In addition, approximately 50% of basolaterally internalized transferrin, a basolateral recycling protein, has access to this apical endosomal compartment and is efficiently recycled back to the basolateral surface. Microtubules are required for the organization of the apical endosomal compartment and it is dispersed in nocodazole-treated cells. Moreover, this compartment is largely inaccessible to fluid-phase markers added to either pole of the cell, and therefore seems analogous to the recycling endosome described in nonpolarized cells. We propose a model in which transcytosis is not a specialized pathway that uses unique transcytotic vesicles, but rather combines portions of pathways used by non-transcytosing molecules.
传统上，人们认为聚合免疫球蛋白受体及其配体IgA是从基底外侧早期内体被分选到直接与顶端质膜融合的转胞吞小泡中。相比之下，我们发现，在MDCK细胞中，IgA是从基底外侧内体转运至顶端内体，然后才到达顶端细胞表面。当从MDCK细胞的基底外侧表面内化时，发现IgA在顶端质膜下方的一个区室中积累，这个区室可被两种顶端添加的膜标记物所接触：抗分泌成分Fab片段，以及从细胞生物素化的顶端极内化的抗生物素蛋白。这种积累发生在顶端存在胰蛋白酶的情况下，胰蛋白酶可阻止配体从顶端细胞表面内化。通过对二氨基联苯胺密度转移测定法的改进，我们估计约80%从基底外侧内化的IgA存在于顶端内体区室中。此外，约50%从基底外侧内化的转铁蛋白（一种基底外侧循环蛋白）可进入这个顶端内体区室，并有效地循环回到基底外侧表面。微管是顶端内体区室组织所必需的，在诺考达唑处理的细胞中它会分散。此外，这个区室在很大程度上无法接触添加到细胞任一极的液相标记物，因此似乎类似于在非极化细胞中描述过的循环内体。我们提出一个模型，其中转胞吞作用不是使用独特转胞吞小泡的特殊途径，而是结合了非转胞吞分子所使用途径的部分过程。
