Figure 2: Hypothetical scheme showing the proximal tubular handling of albumin and other proteins. Albumin (blue) and other proteins (yellow), afterbinding to megalin/cubilin complex on the brush border of proximal tubular cells, are internalized, via clathrin-coated vesicles, that fuse to early endosomes. Acidification of these endosomes causes dissociation of albumin from the megalin/cubilin complex, but, at pH 5-6, albumin binds to FcRn, and undergoes transcytosis. The megalin/cubilin complex is recycled to the apical brushborder membrane, while unbound albumin and other proteins are transferred to lysosomes for degradation by cathepsin B and other lysosomal proteases. Degradation products are exocytosed into the tubular lumen, and excreted in the urine.