Rho GTPase Recognition by C3 Exoenzyme Based on C3-RhoA Complex Structure.

C3 exoenzyme is a mono-ADP-ribosyltransferase (ART) that catalyzes transfer of an ADP-ribose moiety from NAD(+) to Rho GTPases. C3 has long been used to study the diverse regulatory functions of Rho GTPases. How C3 recognizes its substrate and how ADP-ribosylation proceeds are still poorly understood. Crystal structures of C3-RhoA complex reveal that C3 recognizes RhoA via the switch I, switch II, and interswitch regions. In C3-RhoA(GTP) and C3-RhoA(GDP), switch I and II adopt the GDP and GTP conformations, respectively, which explains why C3 can ADP-ribosylate both nucleotide forms. Based on structural information, we successfully changed Cdc42 to an active substrate with combined mutations in the C3-Rho GTPase interface. Moreover, the structure reflects the close relationship among Gln-183 in the QXE motif (C3), a modified Asn-41 residue (RhoA) and NC1 of NAD(H), which suggests that C3 is the prototype ART. These structures show directly for the first time that the ARTT loop is the key to target protein recognition, and they also serve to bridge the gaps among independent studies of Rho GTPases and C3.

Arginine ADP-ribosylation mechanism based on structural snapshots of iota-toxin and actin complex.

Clostridium perfringens iota-toxin (Ia) mono-ADP ribosylates Arg177 of actin, leading to cytoskeletal disorganization and cell death. To fully understand the reaction mechanism of arginine-specific mono-ADP ribosyl transferase, the structure of the toxin-substrate protein complex must be characterized. Recently, we solved the crystal structure of Ia in complex with actin and the nonhydrolyzable NAD(+) analog ßTAD (thiazole-4-carboxamide adenine dinucleotide); however, the structures of the NAD(+)-bound form (NAD(+)-Ia-actin) and the ADP ribosylated form [Ia-ADP ribosylated (ADPR)-actin] remain unclear. Accidentally, we found that ethylene glycol as cryo-protectant inhibits ADP ribosylation and crystallized the NAD(+)-Ia-actin complex. Here we report high-resolution structures of NAD(+)-Ia-actin and Ia-ADPR-actin obtained by soaking apo-Ia-actin crystal with NAD(+) under different conditions. The structures of NAD(+)-Ia-actin and Ia-ADPR-actin represent the pre- and postreaction states, respectively. By assigning the ßTAD-Ia-actin structure to the transition state, the strain-alleviation model of ADP ribosylation, which we proposed previously, is experimentally confirmed and improved. Moreover, this reaction mechanism appears to be applicable not only to Ia but also to other ADP ribosyltransferases.

Crystallization and preliminary X-ray diffraction studies of a surface mutant of the middle domain of PB2 from human influenza A (H1N1) virus.

In the last hundred years, four influenza pandemics have been experienced, beginning with that in Spain in 1918. Influenza A virus causes severe pneumonia and its RNA polymerase is an important target for drug design. The influenza A (H1N1) virus has eight ribonucleoprotein complexes, which are composed of viral RNA, RNA polymerases and nucleoproteins. PB2 forms part of the RNA polymerase complex and plays an important role in binding to the cap structure of host mRNA. The middle domain of PB2 includes a cap-binding site. The structure of PB2 from H1N1 complexed with m(7)GTP has not been reported. Plate-like crystals of the middle domain of PB2 from H1N1 were obtained, but the quality of these crystals was not good. An attempt was made to crystallize the middle domain of PB2 complexed with m(7)GTP using a soaking method; however, electron density for m(7)GTP was not observed on preliminary X-ray diffraction analysis. This protein has hydrophobic residues on its surface and is stable in the presence of high salt concentrations. To improve the solubility, a surface double mutant (P453H and I471T) was prepared. These mutations change the surface electrostatic potential drastically. The protein was successfully prepared at a lower salt concentration and good cube-shaped crystals were obtained using this protein. Here, the crystallization and preliminary X-ray diffraction analysis of this mutant of the middle domain of PB2 are reported.

Conformational polymorphism of m7GTP in crystal structure of the PB2 middle domain from human influenza A virus.

Influenza pandemics with human-to-human transmission of the virus are of great public concern. It is now recognized that a number of factors are necessary for human transmission and virulence, including several key mutations within the PB2 subunit of RNA-dependent RNA polymerase. The structure of the middle domain in PB2 has been revealed with or without m(7)GTP, thus the middle domain is considered to be novel target for structure-based drug design. Here we report the crystal structure of the middle domain of H1N1 PB2 with or without m(7)GTP at 1.9 Å and 2.0 Å resolution, respectively, which has two mutations (P453H, I471T) to increase electrostatic potential and solubility. Here we report the m(7)GTP has unique conformation differ from the reported structure. 7-methyl-guanine is fixed in the pocket, but particularly significant change is seen in ribose and triphosphate region: the buried 7-methyl-guanine indeed binds in the pocket forming by H357, F404, E361 and K376 but the triphosphate continues directly to the outer domain. The presented conformation of m(7)GTP may be a clue for the anti-influenza drug-design.