title: Molecular mediators for raft-dependent endocytosis of syndecan-1, a highly conserved, multifunctional receptor.
abstract: Endocytosis via rafts has attracted considerable recent interest, but the molecular mediators remain incompletely characterized. Here, we focused on the syndecan-1 heparan sulfate proteoglycan, a highly conserved, multifunctional receptor that we previously showed to undergo raft-dependent endocytosis upon clustering. Alanine scanning mutagenesis of three to five consecutive cytoplasmic residues at a time revealed that a conserved juxtamembrane motif, MKKK, was the only region required for efficient endocytosis after clustering. Endocytosis of clustered syndecan-1 occurs in two phases, each requiring a kinase and a corresponding cytoskeletal partner. In the initial phase, ligands trigger rapid MKKK-dependent activation of ERK and the localization of syndecan-1 into rafts. Activation of ERK drives the dissociation of syndecan-1 from α-tubulin, a molecule that may act as an anchor for syndecan-1 at the plasma membrane in the basal state. In the second phase, Src family kinases phosphorylate tyrosyl residues within the transmembrane and cytoplasmic regions of syndecan-1, a process that also requires MKKK. Tyrosine phosphorylation of syndecan-1 triggers the robust recruitment of cortactin, which we found to be an essential mediator of efficient actin-dependent endocytosis. These findings represent the first detailed characterization of the molecular events that drive endocytosis of a raft-dependent receptor and identify a novel endocytic motif, MKKK. Moreover, the results provide new tools to study syndecan function and regulation during uptake of its biologically and medically important ligands, such as HIV-1, atherogenic postprandial remnant lipoproteins, and molecules implicated in Alzheimer disease.
authors.affiliation: Division of Endocrinology, Department of Medicine, Temple University School of Medicine, Philadelphia, Pennsylvania 19140.
authors.forename: Keyang
authors.initials: K
authors.lastname: Chen
authors.affiliation: Division of Endocrinology, Department of Medicine, Temple University School of Medicine, Philadelphia, Pennsylvania 19140. Electronic address: kjwilliams@temple.edu.
authors.forename: Kevin Jon
authors.initials: KJ
authors.lastname: Williams
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D016000:Cluster Analysis; D051356:Cortactin / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004705:Endocytosis; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism; D008024:Ligands; D021962:Membrane Microdomains / Q000378:metabolism*; D008969:Molecular Sequence Data; D016296:Mutagenesis; D010766:Phosphorylation; D054730:Protein Interaction Domains and Motifs; D051381:Rats; D017386:Sequence Homology, Amino Acid; D053668:Syndecan-1 / Q000378:metabolism*; D014404:Tubulin / Q000378:metabolism
chemical_list: D051356:Cortactin; D007074:Immunoglobulin G; D008024:Ligands; D053668:Syndecan-1; D014404:Tubulin
keywords: ERK; Lipid Raft; Lipoprotein Receptor; Proteoglycan; Receptor Endocytosis; Src