title: Josephin domain of ataxin-3 contains two distinct ubiquitin-binding sites.
abstract: Joseph-Machado is an incurable neurodegenerative disease caused by toxic aggregation of ataxin-3, a ubiquitin-specific cysteine protease, involved in the ubiquitin-proteasome pathway and known to bind poly-ubiquitin chains of four or more subunits. The enzymatic site resides in the N-terminal josephin domain of ataxin-3. We have characterized the ubiquitin-binding properties of josephin and showed that, unexpectedly, josephin contains two contiguous but distinct ubiquitin-binding sites. One is close to the enzymatic cleft and exploits an induced fit mechanism, which involves a flexible helical hairpin; the other overlaps with the site involved in recognition of HHR23B, a protein involved in delivering proteolytic substrates to the proteasome. To gain a structural description of the system, we had to overcome the nontrivial problem of dealing with a weak ternary complex. This was done by designing josephin mutants, which retain only one binding site and by characterizing the complexes with complementary computational and experimental techniques. The presence of two ubiquitin-binding sites explains how ataxin-3 binds poly-ubiquitin chains and provides new insights into the molecular mechanism of ubiquitin recognition.
authors.affiliation: National Institute for Medical Research, The Ridgeway, London NW7 1AA, UK.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Nicastro
authors.forename: Laura
authors.initials: L
authors.lastname: Masino
authors.forename: Veronica
authors.initials: V
authors.lastname: Esposito
authors.forename: Rajesh P
authors.initials: RP
authors.lastname: Menon
authors.forename: Alfonso
authors.initials: A
authors.lastname: De Simone
authors.forename: Franca
authors.initials: F
authors.lastname: Fraternali
authors.forename: Annalisa
authors.initials: A
authors.lastname: Pastore
mesh_terms: D000067699:Ataxin-3; D001665:Binding Sites; D003198:Computer Simulation; D006801:Humans; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D009154:Mutation; D009419:Nerve Tissue Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D009687:Nuclear Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012097:Repressor Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D025801:Ubiquitin / Q000737:chemistry* / Q000378:metabolism
chemical_list: D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; D012097:Repressor Proteins; D025801:Ubiquitin; C092341:ATXN3 protein, human; D000067699:Ataxin-3