AMN Directs Endocytosis of the Intrinsic Factor-Vitamin B12 Receptor Cubam by Engaging ARH or Dab2

Abstract

Cubam is a multi-ligand receptor involved in dietary uptake of intrinsic factor-vitamin B(12) in the small intestine and reabsorption of various low-molecular-weight proteins (such as albumin, transferrin, apolipoprotein A-I and vitamin D-binding protein) in the kidney. Cubam is composed of two proteins: cubilin and amnionless. Cubilin harbors ligand binding capabilities, while amnionless provides membrane anchorage and potential endocytic capacity via two FXNPXF signals within the cytosolic domain. These signals are similar to the FXNPXY signals found in members of the low-density lipoprotein receptor superfamily, which associate with clathrin-associated sorting proteins, including Disabled-2 (Dab2) and autosomal recessive hypercholesterolemia (ARH), during endocytosis. We therefore investigated the functionality of each amnionless FXNPXF signal and their respective interaction with sorting proteins. By sequential mutation and expression of a panel of amnionless mutants combined with yeast two-hybrid analyses, we demonstrate that the signals are functionally redundant and both are able to mediate endocytosis of cubam through interaction with Dab2 and ARH.