Two classes of molecules - the LDL receptor (LDLr) family and cell-surface heparan sulfate proteoglycans (HSPGs) - are key participants in the transport of hydrophobic nutrients by plasma lipoproteins. We have discovered a novel pathway, in which endocytosis of lipoproteins and other ligands is mediated directly by syndecan HSPGs. Using a chimera, FcR-Synd, that consists of an IgG Fc receptor ectodomain linked to the transmembrane (TM) and cytoplasmic regions of syndecan-1, we found that efficient endocytosis is triggered by clustering of syndecan or the chimera. Clustering causes rapid movement into cholesterol-rich, detergent-insoluble, membrane rafts, and then the actual uptake into the cell requires recruitment of tyrosine kinases and the actin cytoskeleton. Surprisingly, we found that constructs containing either the LDLr TM or the syndecan-1 TM domain localized equally well to rafts upon clustering. Sequence comparisons revealed an unexpected 15- residue consensus between the inner (C-terminal) portions of the syndecan and LDLr TM domains, which was not shared by a protein excluded from rafts. Importantly, this consensus may explain unusual features of the way these two molecules have been shown to process multivalent ligands, such as large apoE-rich remnant lipoproteins. Thus, the central hypothesis of this proposal is that specific motifs of syndecan, including the raft-localizing segment shared with the LDL receptor, direct the sub-cellular trafficking of nutrient-bearing ligands, with specific functional consequences. There are two Aims.
Aim 1 : Detailed definition of novel trafficking motifs in the LDLr gene family and in syndecan.
In Aim 1 a, we will use CHO cells andMcArdle hepatocytes to map determinants of raft localization within the TM domain of the LDLr, other members of the LDLr gene family, and syndecan.
In Aim 1 b, we will map trafficking determinants in the syndecan cytoplasmic tail.
In Aim I c, we will test these determinants in another key cell type, the macrophage, which is of particular interest becauseofits variant endocytic pathway through the LDLr.
Aim 2 : Functional roles for the novel raft-localizing motif in the LDLr transmembrane domain.
In Aim 2 a, we will determine the role of TM raft-localizing motifs from Aim 1 in the marked stimulation of ACAT that occurs in macrophages when the LDLr binds multivalent lipoproteins.
In Aim 2 b, the role of these TM motifs in LDLr-mediated regulation of apoB secretion via re-uptake will be investigated in hepatocytes. These proposed studies will clarify basic mechanisms and functional consequences of these novel endocytic determinants within the LDLr and syndecans, including the role of raft localization during nutrient delivery.
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