title: Rabex-5 protein regulates the endocytic trafficking pathway of ubiquitinated neural cell adhesion molecule L1.
abstract: Ubiquitination of integral membrane proteins is a common posttranslational modification used to mediate endocytosis and endocytic sorting of cell surface proteins in eukaryotic cells. Ubiquitin (Ub)-binding proteins (UBPs) regulate the stability, function, and localization of ubiquitinated cell surface proteins in the endocytic pathway. Here, I report that the immunoglobulin superfamily cell adhesion molecule L1 undergoes ubiquitination and dephosphorylation on the plasma membrane upon L1 antibody-induced clustering, which mimics L1-L1 homophilic binding, and that these modifications are critical for obtaining the maximal rate of internalization and trafficking to the lysosome, but not to the proteasome. Notably, L1 antibody-induced clustering leads to the association of ubiquitinated L1 with Rabex-5, a UBP and guanine nucleotide exchange factor for Rab5, via interaction with the motif interacting with Ub (MIU) domain, but not the A20-type zinc finger domain. This interaction specifically depends on the presence of an Ub moiety on lysine residues in L1. Rabex-5 expression accelerates the internalization rates of L1(WT) and L1(Y1176A), a tyrosine-based motif mutant, but not L1(K11R), an ubiquitination-deficient mutant, leading to the accumulation of ubiquitinated L1 on endosomes. In contrast, RNA interference-mediated knockdown of Rabex-5 impairs the internalizations of L1(WT) and L1(Y1176A), but not L1(K11R) from the plasma membrane. Overall, these results provide a novel mechanistic insight into how Rabex-5 regulates internalization and postendocytic trafficking of ubiquitinated L1 destined for lysosomal degradation.
authors: Laboratory of Neural Membrane Biology, Graduate School of Brain Science, Doshisha University, 1-3 Miyakodani, Kyotanabe, Kyoto 610-0394, Japan. yaikawa@mail.doshisha.ac.jp
authors: Yoshikatsu
authors: Y
authors: Aikawa
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism; D004705:Endocytosis / Q000502:physiology*; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D008247:Lysosomes / Q000235:genetics / Q000378:metabolism; D051379:Mice; D020125:Mutation, Missense; D039842:Neural Cell Adhesion Molecule L1 / Q000235:genetics / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000235:genetics; D059748:Proteolysis; D057149:Ubiquitinated Proteins / Q000235:genetics / Q000378:metabolism*; D054875:Ubiquitination / Q000235:genetics; D016335:Zinc Fingers
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D039842:Neural Cell Adhesion Molecule L1; C108382:RABGEF1 protein, human; C510870:RabGEF1 protein, mouse; D057149:Ubiquitinated Proteins