RESUMO
SEDS family peptidoglycan (PG) glycosyltransferases, RodA and FtsW, require their cognate transpeptidases PBP2 and FtsI (class B penicillin binding proteins) to synthesize PG along the cell cylinder and at the septum, respectively. The activities of these SEDS-bPBPs complexes are tightly regulated to ensure proper cell elongation and division. In Escherichia coli FtsN switches FtsA and FtsQLB to the active forms that synergize to stimulate FtsWI, but the exact mechanism is not well understood. Previously, we isolated an activation mutation in ftsW (M269I) that allows cell division with reduced FtsN function. To try to understand the basis for activation we isolated additional substitutions at this position and found that only the original substitution produced an active mutant whereas drastic changes resulted in an inactive mutant. In another approach we isolated suppressors of an inactive FtsL mutant and obtained FtsWE289G and FtsIK211I and found they bypassed FtsN. Epistatic analysis of these mutations and others confirmed that the FtsN-triggered activation signal goes from FtsQLB to FtsI to FtsW. Mapping these mutations, as well as others affecting the activity of FtsWI, on the RodA-PBP2 structure revealed they are located at the interaction interface between the extracellular loop 4 (ECL4) of FtsW and the pedestal domain of FtsI (PBP3). This supports a model in which the interaction between the ECL4 of SEDS proteins and the pedestal domain of their cognate bPBPs plays a critical role in the activation mechanism.
Assuntos
Proteínas de Bactérias/ultraestrutura , Proteínas de Escherichia coli/ultraestrutura , Proteínas de Membrana/ultraestrutura , Complexos Multiproteicos/ultraestrutura , Proteínas de Ligação às Penicilinas/ultraestrutura , Peptidoglicano Glicosiltransferase/ultraestrutura , Conformação Proteica , Proteínas de Bactérias/química , Proteínas de Bactérias/genética , Proteínas de Escherichia coli/química , Proteínas de Escherichia coli/genética , Proteínas de Membrana/química , Proteínas de Membrana/genética , Modelos Moleculares , Complexos Multiproteicos/química , Complexos Multiproteicos/genética , Proteínas de Ligação às Penicilinas/química , Proteínas de Ligação às Penicilinas/genética , Peptidoglicano/química , Peptidoglicano/genética , Peptidoglicano/ultraestrutura , Peptidoglicano Glicosiltransferase/química , Peptidoglicano Glicosiltransferase/genética , Peptidil Transferases/química , Peptidil Transferases/genética , Peptidil Transferases/ultraestruturaRESUMO
Glycogen branching enzyme (GBE) catalyzes the formation of α-1,6-branching points during glycogenesis by cleaving α-1,4 bonds and making new α-1,6 bonds. Most GBEs belong to the glycoside hydrolase 13 family (GH13), but new GBEs in the GH57 family have been isolated from Archaea. Here, we determined the crystal structure of a GH57 GBE from the hyperthermophilic archaeon Pyrococcus horikoshii (PhGBE) at a resolution of 2.3 Å. PhGBE exhibits both α-1,6-branching activity and endo-α-1,4 hydrolytic activity. PhGBE has a central (ß/α)7-barrel domain that contains an embedded helix domain and an α-helix-rich C-terminal domain. The active-site cleft is located at the interface of the central and C-terminal domains. Amino acid substitution at Trp22, which is separate from the catalytic nucleophilic residue, abolished both enzymatic activities, indicating that Trp22 might be responsible for substrate recognition. We also observed that shortening of the flexible loop near the catalytic residue changed branched chain lengths of the reaction products with increased hydrolytic activity. Taken together, our findings propose a molecular mechanism for how GH57 GBEs exhibit the two activities and where the substrate binds the enzyme.
