RESUMO
Staphylococcal exfoliative toxins (ETs) are glutamyl endopeptidases that specifically cleave the Glu381-Gly382 bond in the ectodomains of desmoglein 1 (Dsg1) via complex action mechanisms. To date, four ETs have been identified in different Staphylococcus aureus strains and ETE is the most recently characterized. The unusual properties of ETs have been attributed to a unique structural feature, i.e., the 180° flip of the carbonyl oxygen (O) of the nonconserved residue 192/186 (ETA/ETE numbering), not conducive to the oxyanion hole formation. We report the crystal structure of ETE determined at 1.61 Å resolution, in which P186(O) adopts two conformations displaying a 180° rotation. This finding, together with free energy calculations, supports the existence of a dynamic transition between the conformations under the tested conditions. Moreover, enzymatic assays showed no significant differences in the esterolytic efficiency of ETE and ETE/P186G, a mutant predicted to possess a functional oxyanion hole, thus downplaying the influence of the flip on the activity. Finally, we observed the formation of ETE homodimers in solution and the predicted homodimeric structure revealed the participation of a characteristic nonconserved loop in the interface and the partial occlusion of the protein active site, suggesting that monomerization is required for enzymatic activity.
Assuntos
Exfoliatinas , Infecções Estafilocócicas , Domínio Catalítico , Exfoliatinas/química , Exfoliatinas/metabolismo , Humanos , Staphylococcus aureus/metabolismoRESUMO
Exfoliative toxins are serine proteases secreted by Staphylococcus aureus that are associated with toxin-mediated staphylococcal syndromes. To date, four different serotypes of exfoliative toxins have been identified and 3 of them (ETA, ETB, and ETD) are linked to human infection. Among these toxins, only the ETD structure remained unknown, limiting our understanding of the structural determinants for the functional differentiation between these toxins. We recently identified an ETD-like protein associated to S. aureus strains involved in mild mastitis in sheep. The crystal structure of this ETD-like protein was determined at 1.95 Å resolution and the structural analysis provide insights into the oligomerization, stability and specificity and enabled a comprehensive structural comparison with ETA and ETB. Despite the highly conserved molecular architecture, significant differences in the composition of the loops and in both the N- and C-terminal α-helices seem to define ETD-like specificity. Molecular dynamics simulations indicate that these regions defining ET specificity present different degrees of flexibility and may undergo conformational changes upon substrate recognition and binding. DLS and AUC experiments indicated that the ETD-like is monomeric in solution whereas it is present as a dimer in the asymmetric unit indicating that oligomerization is not related to functional differentiation among these toxins. Differential scanning calorimetry and circular dichroism assays demonstrated an endothermic transition centered at 52 °C, and an exothermic aggregation in temperatures up to 64 °C. All these together provide insights about the mode of action of a toxin often secreted in syndromes that are not associated with either ETA or ETB.
Assuntos
Exfoliatinas/química , Exfoliatinas/toxicidade , Staphylococcus aureus/química , Staphylococcus aureus/patogenicidade , Animais , Cristalografia por Raios X , Exfoliatinas/classificação , Feminino , Humanos , Modelos Moleculares , Simulação de Dinâmica Molecular , Conformação Proteica , Ovinos , Infecções Estafilocócicas/etiologia , Infecções Estafilocócicas/microbiologia , Eletricidade Estática , Homologia Estrutural de Proteína , SíndromeRESUMO
With a view to obtaining a better understanding of the structural determinants of P1 glutamate specificity in glutamate-specific endopeptidases (GSEs), the active sites and specificity pockets of such enzymes from Bacillus licheniformis (gse-bl), Bacillus subtilis (mpr) and Staphylococcus aureus (v8 protease) were modelled. This approach was extended to the epidermolytic toxins (ETs), responsible for the staphylococcal scalded skin syndrome. We identify a canonical structure for the S1 subsite, composed of H213 and T190, both of which we predict to interact directly with the P1 glutamate. The possible importance of R30 (for gse-bl and mpr) and of the N-terminus (for gse-bl, mpr and v8 protease) was also examined. In the case of mpr, a G193C substitution is predicted to participate in a novel disulphide bridge which stabilizes C193 in such a way as to maintain the oxyanion hole. In v8, the loss or substitution of several important structural components around D102 of the catalytic triad probably explains its reduced catalytic efficiency in comparison with other GSEs. In the case of the epidermolytic toxins K216 may be important for the previously reported phospholipase C-like activity, since the model predicts that it may stabilize the negative charge on the phosphonyl group.