Molecular identification of a novel candidate sorting receptor purified from human brain by receptor-associated protein affinity chromatography

Receptor-associated protein (RAP) is an endoplasmic reticulum/Golgi protein involved in the processing of receptors of the low density lipoprotein receptor family. A similar to 95-kDa membrane glycoprotein, designated gp95/sortilin, was purified from human brain extracts by RAP affinity chromatography and cloned in a human cDNA library. The gene maps to chromosome Ip and encodes an 833-amino acid type I receptor containing an N-terminal furin cleavage site immediately preceding the N terminus determined in the purified protein. Gp95/sortilin is expressed in several tissues including brain, spinal cord, and testis. Gp95/sortilin is not related to the low density lipoprotein receptor family but shows intriguing homologies to established sorting receptors: a 140-amino acid lumenal segment of sortilin representing a hitherto unrecognized type of extracellular module shows extensive homology to corresponding segments in each of the two lumenal domains of yeast Vps10p, and the extreme C terminus of the cytoplasmic tail of sortilin contains the casein kinase phosphorylation consensus site and an adjacent dileucine sorting motif that mediate assembly protein-1 binding and lysosomal sorting of the mannose 6-phosphate receptors. Expression of a chimeric receptor containing the cytoplasmic tail of gp95/sortilin demonstrates evidence that the tail conveys colocalization with the cation-independent mannose-6-phosphate receptor in endosomes and the Golgi compartment.