Lysosomal membrane chaperone required to export cobalamin (vitamin B12) from the lysosome to the cytosol, allowing its conversion to cofactors . Targets ABCD4 transporter from the endoplasmic reticulum to the lysosome . Then forms a complex with lysosomal ABCD4 and cytoplasmic MMACHC to transport cobalamin across the lysosomal membrane . Acts as an adapter protein which plays an important role in mediating and regulating the internalization of the insulin receptor (INSR) (By similarity). Involved in clathrin-mediated endocytosis of INSR via its interaction with adapter protein complex 2 (By similarity). Essential for the initiation of gastrulation and early formation of mesoderm structures during embryogenesis (By similarity).
(Microbial infection) May play a role in the assembly of hepatitis delta virus (HDV).
Subcellular locations: Endoplasmic reticulum membrane, Lysosome membrane, Cell membrane, Cytoplasmic vesicle, Clathrin-coated vesicle
Isoform 3 is expressed in liver.