title: The transmembrane domain and PXXP motifs of ApoE receptor 2 exclude it from carrying out clathrin-mediated endocytosis.
abstract: The low density lipoprotein (LDL) receptor family comprises several proteins with similar structures including the LDL receptor and apoE receptor 2 (apoER2). The human brain expresses two major splice variants of apoER2 mRNA, one of which includes an additional exon that encodes 59 residues in the cytoplasmic domain. This exon is absent from the LDL receptor and contains three proline-rich (PXXP) motifs that may allow apoER2 to function as a signal transducer. To investigate the role of this insert, we took advantage of the well characterized low density lipoprotein receptor pathway. Chimeras comprising the ectodomain and transmembrane domain of the LDL receptor fused to the cytoplasmic domain of apoER2 lacking the PXXP-containing residues are able to mediate clathrin-dependent endocytosis of LDL as effectively as cells expressing the LDL receptor but not if the PXXP insert is present in the protein. Although expressed on the cell surface, the PXXP-containing chimeric receptor is excluded from clathrin vesicles as judged by its failure to co-localize with adaptor protein-2 possibly due to interaction with intracellular adaptors or scaffolding proteins. Chimeras with the transmembrane domain of apoER2, predicted to be longer than that of the LDL receptor by several residues, fail to mediate endocytosis of LDL or to co-localize with adaptor protein-2 regardless of the presence or absence of the PXXP insert. Thus features of apoER2 that distinguish it as a signaling receptor, rather than as an endocytosis receptor like the LDL receptor, reside in or near the transmembrane domain and in the proline-rich motifs.
authors.affiliation: Lipoprotein Group, Medical Research Council Clinical Sciences Centre, Imperial College Faculty of Medicine, London W12 ONN, United Kingdom.
authors.forename: Xi-Ming
authors.initials: XM
authors.lastname: Sun
authors.forename: Anne K
authors.initials: AK
authors.lastname: Soutar
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D002966:Clathrin / Q000502:physiology*; D006224:Cricetinae; D004705:Endocytosis / Q000502:physiology*; D006801:Humans; D026502:LDL-Receptor Related Proteins; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D018110:Receptors, Lipoprotein / Q000737:chemistry / Q000502:physiology*; D017386:Sequence Homology, Amino Acid
chemical_list: D002966:Clathrin; D026502:LDL-Receptor Related Proteins; D018110:Receptors, Lipoprotein; C098773:low density lipoprotein receptor-related protein 8