Structure-based evolution of a promiscuous inhibitor to a selective stabilizer of protein-protein interactions.

The systematic stabilization of protein-protein interactions (PPI) has great potential as innovative drug discovery strategy to target novel and hard-to-drug protein classes. The current lack of chemical starting points and focused screening opportunities limits the identification of small molecule stabilizers that engage two proteins simultaneously. Starting from our previously described virtual screening strategy to identify inhibitors of 14-3-3 proteins, we report a conceptual molecular docking approach providing concrete entries for discovery and rational optimization of stabilizers for the interaction of 14-3-3 with the carbohydrate-response element-binding protein (ChREBP). X-ray crystallography reveals a distinct difference in the binding modes between weak and general inhibitors of 14-3-3 complexes and a specific, potent stabilizer of the 14-3-3/ChREBP complex. Structure-guided stabilizer optimization results in selective, up to 26-fold enhancement of the 14-3-3/ChREBP interaction. This study demonstrates the potential of rational design approaches for the development of selective PPI stabilizers starting from weak, promiscuous PPI inhibitors.

5-Hydroxymethylcytosine in E-box motifs ACAT|GTG and ACAC|GTG increases DNA-binding of the B-HLH transcription factor TCF4.

We evaluated DNA binding of the B-HLH family members TCF4 and USF1 using protein binding microarrays (PBMs) containing double-stranded DNA probes with cytosine on both strands or 5-methylcytosine (5mC) or 5-hydroxymethylcytosine (5hmC) on one DNA strand and cytosine on the second strand. TCF4 preferentially bound the E-box motif (CAN|NTG) with strongest binding to the 8-mer CAG|GTGGT. 5mC uniformly decreases DNA binding of both TCF4 and USF1. The bulkier 5hmC also inhibited USF1 binding to DNA. In contrast, 5hmC dramatically enhanced TCF4 binding to E-box motifs ACAT|GTG and ACAC|GTG, being better bound than any 8-mer containing cytosine. Examination of X-ray structures of the closely related TCF3 and USF1 bound to DNA suggests TCF3 can undergo a conformational shift to preferentially bind to 5hmC while the USF1 basic region is bulkier and rigid precluding a conformation shift to bind 5hmC. These results greatly expand the regulatory DNA sequence landscape bound by TCF4.

Low resolution structural models of the basic helix-loop-helix leucine zipper domain of upstream stimulatory factor 1 and its complexes with DNA from small angle X-ray scattering data.

The upstream stimulatory factor 1 (USF1) belongs to the basic helix-loop-helix leucine zipper (b/HLH/Z) transcription factor family, recognizing the CACGTG DNA motive as a dimer and playing an important role in the regulation of transcription in a variety of cellular and viral promoters. In this study we investigate the USF1 b/HLH/Z domain and its complexes with DNA by small angle x-ray scattering. We present low resolution structural models of monomeric b/HLH/Z USF1 in the absence of DNA and USF1 dimeric (b/HLH/Z)(2)-DNA and tetrameric (b/HLH/Z)(4)-DNA(2) complexes. The data reveal a concentration-dependent USF1 dimer (b/HLH/Z)(2)-DNA-tetramer (b/HLH/Z)(4)-DNA(2) equilibrium. The ability of b/HLH/Z USF1 to form a tetrameric assembly on two distant DNA binding sites as a consequence of increased protein concentration suggest a USF1 concentration-dependant mechanism of transcription activation involving DNA loop formation.