Molecular Identification of a Novel Candidate Sorting Receptor Purified from Human Brain by Receptor-associated Protein Affinity Chromatography

Abstract

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...

Protein sorting gone wrong – VPS10P domain receptors in cardiovascular and metabolic diseases

“…These studies led to the cloning of two type-1 transmembrane proteins termed sortilin 4 and sortilin-related receptor with A-type repeats (SORLA, also known as LR11) 5,6 . Although SORLA and sortilin bound apolipoproteins, they did not share much structural similarity to prototypical lipoprotein receptors of the LDLR gene family (Fig.…”

Neurotrophins and B-cell malignancies

“…It was later discovered to also play a role in neurotrophin signaling [96]. Sortilin cannot induce intracellular signalling alone as it lacks a cytoplasmic catalytic domain.…”

La sortiline : une protéine associée à de multiples fonctions

“…Cependant un phé-nomène de désynchronisation a été observé, entre les différents organes d'un même animal lésé ainsi qu'entre les cultures d'un même organe provenant de différents individus lésés. La contradiction entre ces résultats et ceux ayant montré la perte d'expression rythmique dans des conditions expérimentales similaires [5,6] protéine de 40 kDa qui a servi à purifier et à cloner pour la première fois le gène codant pour la sortiline [1]. Puis, la neurotensine (NT), un neuropeptide de 13 acides aminés grâce auquel nous avons purifié par chromatographie d'affinité l'un des récepteur de la NT, le NTR3, qui s'avéra être identique à la sortiline [2].…”

“…Les propriétés décrites ci-dessus sont la conséquence de la présence de la sortiline à la surface cellulaire. Cependant, cette expression membranaire ne correspond qu'à environ 10 % du contenu cellulaire en sortiline, le reste de la protéine étant localisé au niveau du Golgi [1]. C'est cette fraction intracellulaire d'environ 90 % qui est responsable de l'autre fonction de la sortiline : la régulation du trafic intracellulaire.…”

“…Plusieurs de ces cellules n'exprimant pas le NTR1, le partenaire éventuel de la sortiline reste à identifier. [1], ont suggéré que le strontium pouvait avoir des propriétés anaboliques osseuses, donc une utilité potentielle pour le traitement de l'ostéoporose. Ces observations ont incité les chimistes des Laboratoires Servier à synthétiser une nouvelle molé-cule organique contenant deux atomes de strontium stable, le ranélate de strontium (RS).…”