Figure 9.

Schematic for transcytosis in proximal tubular cells. (1) From the tubular lumen, albumin (alb, orange), IgG (yellow), and other proteins (blue) are bound to the brushborder of proximal tubular cells through the cubilin/megalin complex. (2 and 3) Acidification in early endosomes results in a release of the bound proteins from the cubilin/megalin complex. At a pH of 5–6, albumin and IgG bind to FcRn, whereas other proteins do not. (4a) The cubilin/megalin complex is recycled back to the apical brushborder. (4b) FcRn-bound proteins are sorted to the basolateral aspect of the cell from where they are released. (4c) The remaining proteins are destined for lysosomal degradation.