Highly specific intracellular ubiquitination of a small molecule.

Ubiquitin is a small, highly conserved protein that acts as a posttranslational modification in eukaryotes. Ubiquitination of proteins frequently serves as a degradation signal, marking them for disposal by the proteasome. Here, we report a novel small molecule from a diversity-oriented synthesis library, BRD1732, that is directly ubiquitinated in cells, resulting in dramatic accumulation of inactive ubiquitin monomers and polyubiquitin chains causing broad inhibition of the ubiquitin-proteasome system. Ubiquitination of BRD1732 and its associated cytotoxicity are stereospecific and dependent upon two homologous E3 ubiquitin ligases, RNF19A and RNF19B. Our finding opens the possibility for indirect ubiquitination of a target through a ubiquitinated bifunctional small molecule, and more broadly raises the potential for posttranslational modification in trans.

Direct small-molecule inhibitors of KRAS: from structural insights to mechanism-based design.

KRAS is the most frequently mutated oncogene in human cancer. In addition to holding this distinction, unsuccessful attempts to target this protein have led to the characterization of RAS as 'undruggable'. However, recent advances in technology and novel approaches to drug discovery have renewed hope that a direct KRAS inhibitor may be on the horizon. In this Review, we provide an in-depth analysis of the structure, dynamics, mutational activation and inactivation, and signalling mechanisms of RAS. From this perspective, we then consider potential mechanisms of action for effective RAS inhibitors. Finally, we examine each of the many recent reports of direct RAS inhibitors and discuss promising avenues for further development.

A Ras-like domain in the light intermediate chain bridges the dynein motor to a cargo-binding region.

Cytoplasmic dynein, a microtubule-based motor protein, transports many intracellular cargos by means of its light intermediate chain (LIC). In this study, we have determined the crystal structure of the conserved LIC domain, which binds the motor heavy chain, from a thermophilic fungus. We show that the LIC has a Ras-like fold with insertions that distinguish it from Ras and other previously described G proteins. Despite having a G protein fold, the fungal LIC has lost its ability to bind nucleotide, while the human LIC1 binds GDP preferentially over GTP. We show that the LIC G domain binds the dynein heavy chain using a conserved patch of aromatic residues, whereas the less conserved C-terminal domain binds several Rab effectors involved in membrane transport. These studies provide the first structural information and insight into the evolutionary origin of the LIC as well as revealing how this critical subunit connects the dynein motor to cargo.