title: MEX is a testis-specific E3 ubiquitin ligase that promotes death receptor-induced apoptosis.
abstract: In the present study, we report the identification and characterization of MEX (MEKK1-related protein X), a protein with homology to MEKK1 that is expressed uniquely in the testis. MEX is comprises four putative zinc-binding domains including an N-terminal SWIM (SWI2/SNF2 and MuDR) domain of unknown function and two RING (really interesting new gene) fingers separated by a ZZ zinc finger domain. Biochemical analyses revealed that MEX is self-ubiquitinated and targeted for degradation through the proteasome pathway. MEX can act as an E3, Ub (ubiquitin) ligase, through the E2, Ub-conjugating enzymes UbcH5a, UbcH5c or UbcH6. A region of MEX that contains the RING fingers and the ZZ zinc finger was required for interaction with UbcH5a and MEX self-association, whereas the SWIM domain was critical for MEX ubiquitination. The expression of MEX promoted apoptosis that was induced through Fas, DR (death receptor) 3 and DR4 signalling, but not that mediated by the BH3 (Bcl-2 homology 3)-only protein BimEL or the chemotherapeutic drug adriamycin. The enhancement of apoptosis by MEX required a functional SWIM domain, suggesting that MEX ubiquitination is critical for the enhancement of apoptosis. These results indicate that MEX acts as an E3 Ub ligase, an activity that is dependent on the SWIM domain and suggest a role for MEX in the regulation of death receptor-induced apoptosis in the testes.
authors.affiliation: Department of Pathology and Comprehensive Cancer Center, The University of Michigan Medical School, Ann Arbor, Michigan 48109, USA.
authors.forename: Yasumasa
authors.initials: Y
authors.lastname: Nishito
authors.forename: Mizuho
authors.initials: M
authors.lastname: Hasegawa
authors.forename: Naohiro
authors.initials: N
authors.lastname: Inohara
authors.forename: Gabriel
authors.initials: G
authors.lastname: Núñez
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D006801:Humans; D033862:Iron-Binding Proteins / Q000378:metabolism; D008297:Male; D051379:Mice; D008969:Molecular Sequence Data; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D017434:Protein Structure, Tertiary; D051381:Rats; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; D018124:Receptors, Tumor Necrosis Factor / Q000502:physiology*; D053219:Receptors, Tumor Necrosis Factor, Member 25; D013737:Testis / Q000201:enzymology*; D044763:Ubiquitin-Conjugating Enzymes / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000737:chemistry / Q000502:physiology*; D016335:Zinc Fingers / Q000502:physiology; D019014:fas Receptor / Q000502:physiology
chemical_list: D033862:Iron-Binding Proteins; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; D018124:Receptors, Tumor Necrosis Factor; D053219:Receptors, Tumor Necrosis Factor, Member 25; C105190:TNFRSF10A protein, human; C506033:TNFRSF25 protein, human; C506034:Tnfrsf25 protein, mouse; D019014:fas Receptor; C097308:UBE2D1 protein, human; D044763:Ubiquitin-Conjugating Enzymes; D044767:Ubiquitin-Protein Ligases; C510855:Zswim2 protein, mouse; D046988:Proteasome Endopeptidase Complex