Receptor-associated protein (RAP) is an endoplasmic reticulum/Golgi protein involved in the processing of receptors of the low density lipoprotein receptor family. A ϳ95-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 1p 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.Sorting of newly synthesized lysosomal enzymes from the Golgi compartment to late endosomes in eukaryotic cells is a sophisticated transport process involving specific sorting receptors in the trans-Golgi network. In mammals, the 46-and 275-kDa mannose-6-phosphate (M6P) 1 receptors are the known sorting receptors that bind to phosphorylated mannose residues in lysosomal hydrolases (1). In yeast, a M6P-independent sorting pathway has been demonstrated by identification of the vacuolar protein-sorting 10 protein (Vps10p) (2) and a highly homologous protein encoded by the yeast VTH2 gene (3). Both are capable of targeting yeast carboxypeptidase Y to lysosomes (2, 3). Mammalian counterparts to these sorting receptors have so far not been identified. However, studies of I-cell disease patients suggest that mammals may sort lysosomal enzymes by alternative mechanisms (4 -9). The 40-kDa endoplasmic reticulum/Golgi receptor-associated protein (RAP) assists folding and processing of the cysteine-rich low density lipoprotein (LDL) receptor class A repeats in receptors of the LDL receptor family (10 -13). In addition to the high affinity binding to the LDL receptor family proteins and the newly identified LDL receptor type A repeat containing receptor sorLA/LR11 (14, 15), RAP binds calmodulin and is phosphorylated by calmodulin-dependent kinase II and casein kinase II (16). Recently, independent observations have shown the binding of RAP to an approximately 100-kDa protein expressed in osteosarcoma (17) and Chinese hamster ovary cells (18).In the present study we have identified, pu...

The urokinase-type plasminogen activator receptor (uPAR) plays an important role on the cell surface in mediating extracellular degradative processes and formation of active TGF-β, and in nonproteolytic events such as cell adhesion, migration, and transmembrane signaling. We have searched for mechanisms that determine the cellular location of uPAR and may participate in its disposal. When using purified receptor preparations, we find that uPAR binds to the cation-independent, mannose 6-phosphate/insulin-like growth factor–II (IGF-II) receptor (CIMPR) with an affinity in the low micromolar range, but not to the 46-kD, cation-dependent, mannose 6-phosphate receptor (CDMPR). The binding is not perturbed by uPA and appears to involve domains DII + DIII of the uPAR protein moiety, but not the glycosylphosphatidylinositol anchor. The binding occurs at site(s) on the CIMPR different from those engaged in binding of mannose 6-phosphate epitopes or IGF-II. To evaluate the significance of the binding, immunofluorescence and immunoelectron microscopy studies were performed in transfected cells, and the results show that wild-type CIMPR, but not CIMPR lacking an intact sorting signal, modulates the subcellular distribution of uPAR and is capable of directing it to lysosomes. We conclude that a site within CIMPR, distinct from its previously known ligand binding sites, binds uPAR and modulates its subcellular distribution.

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