4nql | pdb_00004nql
The crystal structure of the DUB domain of AMSH orthologue, Sst2 from S. pombe, in complex with lysine 63-linked diubiquitin
Structural highlights
Function[SST2_SCHPO] Zinc metalloprotease that specifically cleaves 'Lys-63'-linked polyubiquitin chains. Does not cleave 'Lys-48'-linked polyubiquitin chains (By similarity). Plays a role in the multivesicular body (MVB) sorting pathway. Required for ubiquitin-dependent sorting of proteins into the endosome and subsequent trafficking to the vacuole. May regulate MVB sorting through deubiquitination of ubiquitinated ESCRT proteins.[1] [UBC_MOUSE] Ubiquitin: Exists either covalently attached to another protein, or free (unanchored). When covalently bound, it is conjugated to target proteins via an isopeptide bond either as a monomer (monoubiquitin), a polymer linked via different Lys residues of the ubiquitin (polyubiquitin chains) or a linear polymer linked via the initiator Met of the ubiquitin (linear polyubiquitin chains). Polyubiquitin chains, when attached to a target protein, have different functions depending on the Lys residue of the ubiquitin that is linked: Lys-6-linked may be involved in DNA repair; Lys-11-linked is involved in ERAD (endoplasmic reticulum-associated degradation) and in cell-cycle regulation; Lys-29-linked is involved in lysosomal degradation; Lys-33-linked is involved in kinase modification; Lys-48-linked is involved in protein degradation via the proteasome; Lys-63-linked is involved in endocytosis, DNA-damage responses as well as in signaling processes leading to activation of the transcription factor NF-kappa-B. Linear polymer chains formed via attachment by the initiator Met lead to cell signaling. Ubiquitin is usually conjugated to Lys residues of target proteins, however, in rare cases, conjugation to Cys or Ser residues has been observed. When polyubiquitin is free (unanchored-polyubiquitin), it also has distinct roles, such as in activation of protein kinases, and in signaling.[2] Publication Abstract from PubMedAMSH, a conserved zinc metallo deubiquitinase, controls downregulation and degradation of cell-surface receptors mediated by the endosomal sorting complexes required for transport (ESCRT) machinery. It displays high specificity toward the Lys63-linked polyubiquitin chain, which is used as a signal for ESCRT-mediated endosomal-lysosomal sorting of receptors. Herein, we report the crystal structures of the catalytic domain of AMSH orthologue Sst2 from fission yeast, its ubiquitin (product)-bound form, and its Lys63-linked diubiquitin (substrate)-bound form at 1.45, 1.7, and 2.3 A, respectively. The structures reveal that the P-side product fragment maintains nearly all the contacts with the enzyme as seen with the P portion (distal ubiquitin) of the Lys63-linked diubiquitin substrate, with additional coordination of the Gly76 carboxylate group of the product with the active-site Zn(2+). One of the product-bound structures described herein is the result of an attempt to cocrystallize the diubiquitin substrate bound to an active site mutant presumed to render the enzyme inactive, instead yielding a cocrystal structure of the enzyme bound to the P-side ubiquitin fragment of the substrate (distal ubiquitin). This fragment was generated in situ from the residual activity of the mutant enzyme. In this structure, the catalytic water is seen placed between the active-site Zn(2+) and the carboxylate group of Gly76 of ubiquitin, providing what appears to be a snapshot of the active site when the product is about to depart. Comparison of this structure with that of the substrate-bound form suggests the importance of dynamics of a flexible flap near the active site in catalysis. The crystal structure of the Thr319Ile mutant of the catalytic domain of Sst2 provides insight into structural basis of microcephaly capillary malformation syndrome. Isothermal titration calorimetry yields a dissociation constant (KD) of 10.2 +/- 0.6 muM for the binding of ubiquitin to the enzyme, a value comparable to the KM of the enzyme catalyzing hydrolysis of the Lys63-linked diubiquitin substrate ( approximately 20 muM). These results, together with the previously reported observation that the intracellular concentration of free ubiquitin ( approximately 20 muM) exceeds that of Lys63-linked polyubiquitin chains, imply that the free, cytosolic form of the enzyme remains inhibited by being tightly bound to free ubiquitin. We propose that when AMSH associates with endosomes, inhibition would be relieved because of ubiquitin binding domains present on its endosomal binding partners that would shift the balance toward better recognition of polyubiquitin chains via the avidity effect. Insights into the Mechanism of Deubiquitination by JAMM Deubiquitinases from Cocrystal Structures of the Enzyme with the Substrate and Product.,Shrestha RK, Ronau JA, Davies CW, Guenette RG, Strieter ER, Paul LN, Das C Biochemistry. 2014 May 20;53(19):3199-217. doi: 10.1021/bi5003162. Epub 2014 May , 9. PMID:24787148[3] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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