The presynaptic protein alpha-synuclein has been implicated in the pathogenesis of Parkinson's disease. First, two missense mutations A30P and A53T cause inheritable early onset Parkinson's disease in some families. Secondly, alpha-synuclein is present in Lewy bodies of affected nerve cells in the predominant sporadic type of Parkinson's disease as well as in dementia with Lewy bodies. We demonstrate in the rat optic system that a portion of alpha-synuclein is carried by the vesicle-moving fast component of axonal transport and that it binds to rat brain vesicles through its amino-terminal repeat region. We find alpha-synuclein with the A30P mutation of familial Parkinson's disease devoid of vesicle-binding activity and propose that mutant alpha-synuclein may accumulate, leading to assembly into Lewy body filaments.

Sortilin belongs to a growing family of multiligand type‐1 receptors with homology to the yeast receptor Vps10p. Based on structural features and sortilin's intracellular predominance, we have proposed it to be a sorting receptor for ligands in the synthetic pathway as well as on the cell membrane. To test this hypothesis we examine here the cellular trafficking of chimeric receptors containing constructs of the sortilin tail. We report that sorting signals conforming to YXXΦ and dileucine motifs mediate rapid endocytosis of sortilin chimeras, which subsequently travel to the trans ‐Golgi network, showing little or no recycling. Furthermore, we found that cation‐independent mannose 6‐phosphate receptor (MPR300)–sortilin chimeras, expressed in mannose 6‐phosphate receptor knockout cells, were almost as efficient as MPR300 itself for transport of newly synthesized β‐hexosaminidase and β‐glucuronidase to lysosomes, and established that the sortilin tail contains potent signals for Golgi–endosome sorting. Finally, we provide evidence suggesting that sortilin is the first example of a mammalian receptor targeted by the recently described GGA family of cytosolic sorting proteins, which condition the Vps10p‐mediated sorting of yeast carboxypeptidase Y.

Receptor-associated protein (RAP) is an endoplasmic reticulum/Golgi protein involved in the processing of receptors of the low density lipoprotein receptor family. A approximately 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 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 mannose6-phosphate receptor in endosomes and the Golgi compartment.

α-Synuclein has been implicated in the pathogenesis of several neurodegenerative disorders based on the direct linking of missense mutations in α-synuclein to autosomal dominant Parkinson’s disease and its presence in Lewy-like lesions. To gain insight into α-synuclein functions, we have investigated whether it binds neuronal proteins and modulates their functional state. The microtubule-associated protein tau was identified as a ligand by α-synuclein affinity chromatography of human brain cytosol. Direct binding assays using ¹²⁵I-labeled human tau40 demonstrated a reversible binding with a IC₅₀ about 50 pm. The interacting domains were localized to the C terminus of α-synuclein and the microtubule binding region of tau as determined by protein fragmentation and the use of recombinant peptides. High concentrations of tubulin inhibited the binding between tau and α-synuclein. Functionally, α-synuclein stimulated the protein kinase A-catalyzed phosphorylation of tau serine residues 262 and 356 as determined using a phospho-epitope-specific antibody. We propose that α-synuclein modulates the phosphorylation of soluble axonal tau and thereby indirectly affects the stability of axonal microtubules.

Steroid hormones are central regulators of a variety of biological processes. According to the free hormone hypothesis, steroids enter target cells by passive diffusion. However, recently we demonstrated that 25(OH) vitamin D ₃ complexed to its plasma carrier, the vitamin D-binding protein, enters renal proximal tubules by receptor-mediated endocytosis. Knockout mice lacking the endocytic receptor megalin lose 25(OH) vitamin D ₃ in the urine and develop bone disease. Here, we report that cubilin, a membrane-associated protein colocalizing with megalin, facilitates the endocytic process by sequestering steroid–carrier complexes on the cellular surface before megalin-mediated internalization of the cubilin-bound ligand. Dogs with an inherited disorder affecting cubilin biosynthesis exhibit abnormal vitamin D metabolism. Similarly, human patients with mutations causing cubilin dysfunction exhibit urinary excretion of 25(OH) vitamin D ₃ . This observation identifies spontaneous mutations in an endocytic receptor pathway affecting cellular uptake and metabolism of a steroid hormone.