title: Rabex-5 is a Rab22 effector and mediates a Rab22-Rab5 signaling cascade in endocytosis.
abstract: Rabex-5 targets to early endosomes and functions as a guanine nucleotide exchange factor for Rab5. Membrane targeting is critical for Rabex-5 to activate Rab5 on early endosomes in the cell. Here, we report the identification of Rab22 as a binding site on early endosomes for direct recruitment of Rabex-5 and activation of Rab5, establishing a Rab22-Rab5 signaling relay to promote early endosome fusion. Rab22 in guanosine 5'-O-(3-thio)triphosphate-loaded form, but not guanosine diphosphate-loaded form, binds to the early endosomal targeting domain (residues 81-230) of Rabex-5 in pull-down assays. Rabex-5 targets to Rab22-containing early endosomes, and Rab22 knockdown by short hairpin RNA abrogates the membrane targeting of Rabex-5 in the cell. In addition, coexpression of Rab22 and Rab5 shows synergistic enlargement of early endosomes, and this synergy is dependent on Rabex-5, providing further support for the collaboration of the two Rab GTPases in regulation of endosome dynamics. This novel Rab22-Rabex-5-Rab5 cascade is functionally important for the endocytosis and degradation of epidermal growth factor.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA.
authors.forename: Huaiping
authors.initials: H
authors.lastname: Zhu
authors.forename: Zhimin
authors.initials: Z
authors.lastname: Liang
authors.forename: Guangpu
authors.initials: G
authors.lastname: Li
mesh_terms: D000818:Animals; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D004705:Endocytosis / Q000502:physiology*; D011992:Endosomes / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D011485:Protein Binding; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D020691:rab GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D020696:rab5 GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D020662:Guanine Nucleotide Exchange Factors; C484191:RAB22A protein, human; D034741:RNA, Small Interfering; C510870:RabGEF1 protein, mouse; D011993:Recombinant Fusion Proteins; D020691:rab GTP-Binding Proteins; D020696:rab5 GTP-Binding Proteins