Nailing Down a Notoriously Elusive Cancer Target: Direct Inhibition of MYC by a Covalent Small Molecule.

Direct inhibition of the transcription factor MYC is widely recognized as one of the thorniest challenges in cancer drug discovery. In this issue of Cell Chemical Biology, Boike et al. (2020) discover a covalent MYC inhibitor that selectively targets a single cysteine residue in an unstructured region of the protein.

Drugging Fuzzy Complexes in Transcription.

Transcription factors (TFs) are one of the most promising but underutilized classes of drug targets. The high degree of intrinsic disorder in both the structure and the interactions (i.e., "fuzziness") of TFs is one of the most important challenges to be addressed in this context. Here, we discuss the impacts of fuzziness on transcription factor drug discovery, describing how disorder poses fundamental problems to the typical drug design, and screening approaches used for other classes of proteins such as receptors or enzymes. We then speculate on ways modern biophysical and chemical biology approaches could synergize to overcome many of these challenges by directly addressing the challenges imposed by TF disorder and fuzziness.

Molecular recognition of pre-tRNA by Arabidopsis protein-only Ribonuclease P.

Protein-only ribonuclease P (PRORP) is an enzyme responsible for catalyzing the 5' end maturation of precursor transfer ribonucleic acids (pre-tRNAs) encoded by various cellular compartments in many eukaryotes. PRORPs from plants act as single-subunit enzymes and have been used as a model system for analyzing the function of the metazoan PRORP nuclease subunit, which requires two additional proteins for efficient catalysis. There are currently few molecular details known about the PRORP-pre-tRNA complex. Here, we characterize the determinants of substrate recognition by the single subunit Arabidopsis thaliana PRORP1 and PRORP2 using kinetic and thermodynamic experiments. The salt dependence of binding affinity suggests 4-5 contacts with backbone phosphodiester bonds on substrates, including a single phosphodiester contact with the pre-tRNA 5' leader, consistent with prior reports of short leader requirements. PRORPs contain an N-terminal pentatricopeptide repeat (PPR) domain, truncation of which results in a >30-fold decrease in substrate affinity. While most PPR-containing proteins have been implicated in single-stranded sequence-specific RNA recognition, we find that the PPR motifs of PRORPs recognize pre-tRNA substrates differently. Notably, the PPR domain residues most important for substrate binding in PRORPs do not correspond to positions involved in base recognition in other PPR proteins. Several of these residues are highly conserved in PRORPs from algae, plants, and metazoans, suggesting a conserved strategy for substrate recognition by the PRORP PPR domain. Furthermore, there is no evidence for sequence-specific interactions. This work clarifies molecular determinants of PRORP-substrate recognition and provides a new predictive model for the PRORP-substrate complex.

Crystal structure of O-ethyl N-(eth-oxy-carbon-yl)thio-carbamate.

The title compound, C6H11NO3S, provides entries to novel carbamoyl disulfanes and related compounds of inter-est to our laboratory. The atoms of the central O(C=S)N(C=O)O fragment have an r.m.s. deviation of 0.1077 Šfrom the respective least-squares plane. While several conformational orientations are conceivable, the crystal structure shows only the one in which the carbonyl and the thio-carbonyl moieties are anti to each other across the central conjugated C-N-C moiety. Pairs of 2.54 ŠN-H⋯S=C hydrogen bonds between adjacent mol-ecules form centrosymmetric dimers in the crystal.

Crystal structures of (N-methyl-N-phenyl-amino)(N-methyl-N-phenyl-carbamoyl)sulfide and the corresponding disulfane.

The title compounds, (N-methyl-N-phenyl-amino)(N-methyl-N-phenyl-car-bam-oyl)sulfide, C15H16N2OS, (I), and (N-methyl-N-phenyl-amino)-(N-methyl-N-phenyl-carbamo-yl)disulfane, C15H16N2OS2, (II), are stable derivatives of (chloro-carbon-yl)sulfenyl chloride and (chloro-carbon-yl)disulfanyl chloride, respectively. The torsion angle about the S-S bond in (II) is -92.62 (6)°, which is close to the theoretical value of 90°. In the crystal of (II), non-classical inter-molecular C-H⋯O hydrogen bonds form centrosymmetric cyclic dimers [graph set R 2 (2)(10)], while inter-dimer C-H⋯S inter-actions generate chains extending along the b axis.

Crystal structure of bis-(N-methyl-N-phenyl-amino)-tris-ulfane.

The title compound, C14H16N2S3, crystallized with two independent mol-ecules [(1 a ) and (1 b )] in the asymmetric unit. Both mol-ecules display a pseudo-trans conformation. The two consecutive S-S bond lengths of the tris-ulfane unit of mol-ecule (1 a ) are 2.06 (3) and 2.08 (3) Å, and 2.08 (3) and 2.07 (2) Šfor mol-ecule (1 b ). Torsion angles about each of the two S-S bonds are 86.6 (2) and 87.0 (2)° for (1 a ), and -84.6 (2) and -85.9 (2)° for (1 b ). The core atoms, viz. the N-S-S-S-N moiety, of the two mol-ecules superimpose well if one is inverted on the other, but the phenyl groups do not. Thus, the two units are essentially conformational enanti-omers. In mol-ecule (1 a ), the two phenyl rings are inclined to one another by 86.7 (3)°, and in mol-ecule (1 b ), by 81.1 (3)°. In the crystal, mol-ecules are linked via C-H⋯π inter-actions, forming sheets lying parallel to (010).