Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching.

The flagellar motor drives the rotation of flagellar filaments at hundreds of revolutions per second, efficiently propelling bacteria through viscous media. The motor uses the potential energy from an electrochemical gradient of cations across the cytoplasmic membrane to generate torque. A rapid switch from anticlockwise to clockwise rotation determines whether a bacterium runs smoothly forward or tumbles to change its trajectory. A protein called FliG forms a ring in the rotor of the flagellar motor that is involved in the generation of torque through an interaction with the cation-channel-forming stator subunit MotA. FliG has been suggested to adopt distinct conformations that induce switching but these structural changes and the molecular mechanism of switching are unknown. Here we report the molecular structure of the full-length FliG protein, identify conformational changes that are involved in rotational switching and uncover the structural basis for the formation of the FliG torque ring. This allows us to propose a model of the complete ring and switching mechanism in which conformational changes in FliG reverse the electrostatic charges involved in torque generation.

Mechanism of CDK5/p25 binding by CDK inhibitors.

The cyclin-dependent kinases (CDK) CDK1, CDK2, CDK4, and CDK6 are serine/threonine protein kinases targeted in cancer therapy due to their role in cell cycle progression. The postmitotic CDK5 is involved in biological pathways important for neuronal migration and differentiation. CDK5 represents an attractive pharmacological target as its deregulation is implicated in various neurodegenerative diseases such as Alzheimer's, Parkinson's, and Niemann-Pick type C diseases, ischemia, and amyotrophic lateral sclerosis. We have generated an improved crystal form of CDK5 in complex with p25, a segment of the p35 neuronal activator. The crystals were used to solve the structure of CDK5/p25 with (R)-roscovitine and aloisine at a resolution of 2.2 and 2.3 A, respectively. The structure of CDK5/p25/roscovitine provides a rationale for the preference of CDK5 for the R over the S stereoisomer. Furthermore, roscovitine stabilized an unusual collapsed conformation of the glycine-rich loop, an important site of CDK regulation, and we report an investigation of the effects of glycine-rich loop phosphorylation on roscovitine binding. The CDK5/p25 crystals represent a valuable new tool for the identification and optimization of selective CDK inhibitors.

GSK-3-selective inhibitors derived from Tyrian purple indirubins.

Gastropod mollusks have been used for over 2500 years to produce the "Tyrian purple" dye made famous by the Phoenicians. This dye is constituted of mixed bromine-substituted indigo and indirubin isomers. Among these, the new natural product 6-bromoindirubin and its synthetic, cell-permeable derivative, 6-bromoindirubin-3'-oxime (BIO), display remarkable selective inhibition of glycogen synthase kinase-3 (GSK-3). Cocrystal structure of GSK-3beta/BIO and CDK5/p25/indirubin-3'-oxime were resolved, providing a detailed view of indirubins' interactions within the ATP binding pocket of these kinases. BIO but not 1-methyl-BIO, its kinase inactive analog, also inhibited the phosphorylation on Tyr276/216, a GSK-3alpha/beta activation site. BIO but not 1-methyl-BIO reduced beta-catenin phosphorylation on a GSK-3-specific site in cellular models. BIO but not 1-methyl-BIO closely mimicked Wnt signaling in Xenopus embryos. 6-bromoindirubins thus provide a new scaffold for the development of selective and potent pharmacological inhibitors of GSK-3.