title: Papaya glutamine cyclotransferase shows a singular five-fold beta-propeller architecture that suggests a novel reaction mechanism.
abstract: Cyclisation of N-terminal glutamine and/or glutamate to yield pyroglutamate is an essential posttranslational event affecting a plethora of bioactive peptides and proteins. It is directly linked with pathologies ranging from neurodegenerative diseases to inflammation and several types of cancers. The reaction is catalysed by ubiquitous glutaminyl cyclotransferases (QCs), which present two distinct prototypes. Mammalian QCs are zinc-dependent enzymes with an alpha/beta-hydrolase fold. Here we present the 1.6-A-resolution structure of the other prototype, the plant analogue from Carica papaya (PQC). The hatbox-shaped molecule consists of an unusual five-fold beta-propeller traversed by a central channel, a topology that has hitherto been described only for some sugar-binding proteins and an extracellular nucleotidase. The high resistance of the enzyme to denaturation and proteolytic degradation is explained by its architecture, which is uniquely stabilised by a series of tethering elements that confer rigidity. Strikingly, the N-terminus of PQC specifically interacts with residues around the entrance to the central channel of a symmetry-related molecule, suggesting that this location is the putative active site. Cyclisation would follow a novel general-acid/base working mechanism, pivoting around a strictly conserved glutamate. This study provides a lead structure not only for plant QC orthologues, but also for bacteria, including potential human pathogens causing diphtheria, plague and malaria.
authors.affiliation: Institut de Biologia Molecular de Barcelona, C.S.I.C., c/ Jordi Girona, 18-26, E-08034 Barcelona, Spain.
authors.forename: Tibisay
authors.initials: T
authors.lastname: Guevara
authors.forename: Noemí
authors.initials: N
authors.lastname: Mallorquí-Fernández
authors.forename: Raquel
authors.initials: R
authors.lastname: García-Castellanos
authors.forename: Sonia
authors.initials: S
authors.lastname: García-Piqué
authors.forename: Gitte
authors.initials: G
authors.lastname: Ebert Petersen
authors.forename: Conni
authors.initials: C
authors.lastname: Lauritzen
authors.forename: John
authors.initials: J
authors.lastname: Pedersen
authors.forename: José
authors.initials: J
authors.lastname: Arnau
authors.forename: F Xavier
authors.initials: FX
authors.lastname: Gomis-Rüth
authors.forename: Maria
authors.initials: M
authors.lastname: Solà
mesh_terms: D000595:Amino Acid Sequence; D019881:Aminoacyltransferases / Q000737:chemistry* / Q000378:metabolism*; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D029441:Carica / Q000201:enzymology*; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment
chemical_list: D019881:Aminoacyltransferases; C059280:glutaminyl-peptide cyclotransferase; D002118:Calcium