title: The RanBP2 SUMO E3 ligase is neither HECT- nor RING-type.
abstract: Post-translational modification with the ubiquitin-related protein SUMO1 requires the E1 enzyme Aos1-Uba2 and the E2 enzyme Ubc9. Distinct E3 ligases strongly enhance modification of specific targets. The SUMO E3 ligase RanBP2 (also known as Nup358) has no obvious similarity to RING- or HECT-type enzymes. Here we show that RanBP2's 30-kDa catalytic fragment is a largely unstructured protein. Despite two distinct but partially overlapping 79-residue catalytic domains, one of which is sufficient for maximal activity, RanBP2 binds to Ubc9 in a 1:1 stoichiometry. The identification of nine RanBP2 and three Ubc9 side chains that are important for RanBP2-dependent SUMOylation indicates largely hydrophobic interactions. These properties distinguish RanBP2 from all other known E3 ligases, and we speculate that RanBP2 exerts its catalytic effect by altering Ubc9's properties rather than by mediating target interactions.
authors.affiliation: Max-Planck Institute for Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany.
authors.forename: Andrea
authors.initials: A
authors.lastname: Pichler
authors.forename: Puck
authors.initials: P
authors.lastname: Knipscheer
authors.forename: Hisato
authors.initials: H
authors.lastname: Saitoh
authors.forename: Titia K
authors.initials: TK
authors.lastname: Sixma
authors.forename: Frauke
authors.initials: F
authors.lastname: Melchior
mesh_terms: D000595:Amino Acid Sequence; D020134:Catalytic Domain; D018832:Molecular Chaperones; D008969:Molecular Sequence Data; D028861:Nuclear Pore Complex Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D018832:Molecular Chaperones; D028861:Nuclear Pore Complex Proteins; C094493:ran-binding protein 2