Chemical shift perturbation studies of the interactions of the second RNA-binding domain of the Drosophila sex-lethal protein with the transformer pre-mRNA polyuridine tract and 3' splice-site sequences

Process of Fragment-Based Lead Discovery-A Perspective from NMR

Fragment-based lead discovery (FBLD) has proven fruitful during the past two decades for a variety of targets, even challenging protein–protein interaction (PPI) systems. Nuclear magnetic resonance (NMR) spectroscopy plays a vital role, from initial fragment-based screening to lead generation, because of its power to probe the intrinsically weak interactions between targets and low-molecular-weight fragments. Here, we review the NMR FBLD process from initial library construction to lead generation. We describe technical aspects regarding fragment library design, ligand- and protein-observed screening, and protein–ligand structure model generation. For weak binders, the initial hit-to-lead evolution can be guided by structural information retrieved from NMR spectroscopy, including chemical shift perturbation, transferred pseudocontact shifts, and paramagnetic relaxation enhancement. This perspective examines structure-guided optimization from weak fragment screening hits to potent leads for challenging PPI targets.

Three RNA recognition motifs participate in RNA recognition and structural organization by the pro-apoptotic factor TIA-1

T-cell intracellular antigen-1 (TIA-1) regulates developmental and stress-responsive pathways through distinct activities at the levels of alternative pre-mRNA splicing and mRNA translation. The TIA-1 polypeptide contains three RNA recognition motifs (RRMs). The central RRM2 and C-terminal RRM3 associate with cellular mRNAs. The N-terminal RRM1 enhances interactions of a C-terminal Q-rich domain of TIA-1 with the U1-C splicing factor, despite linear separation of the domains in the TIA-1 sequence. Given the expanded functional repertoire of the RRM family, it was unknown whether TIA-1 RRM1 contributes to RNA binding as well as documented protein interactions. To address this question, we used isothermal titration calorimetry and small-angle X-ray scattering to dissect the roles of the TIA-1 RRMs in RNA recognition. Notably, the fas RNA exhibited two binding sites with indistinguishable affinities for TIA-1. Analyses of TIA-1 variants established that RRM1 was dispensable for binding AU-rich fas sites, yet all three RRMs were required to bind a polyU RNA with high affinity. Small-angle X-ray scattering analyses demonstrated a "V" shape for a TIA-1 construct comprising the three RRMs and revealed that its dimensions became more compact in the RNA-bound state. The sequence-selective involvement of TIA-1 RRM1 in RNA recognition suggests a possible role for RNA sequences in regulating the distinct functions of TIA-1. Further implications for U1-C recruitment by the adjacent TIA-1 binding sites of the fas pre-mRNA and the bent TIA-1 shape, which organizes the N- and C-termini on the same side of the protein, are discussed.

NMR studies on functional structures of the AU-rich element-binding domains of Hu antigen C

Hu antigen C (HuC) has three RNA-binding domains (RBDs). The N-terminal two, RBD1 and RBD2, are linked in tandem and bind to the AU-rich elements (AREs) in the 3'-untranslated region of particular mRNAs. The solution structures of HuC RBD1 and RBD2 were determined by NMR methods. The HuC RBD1 and RBD2 structures are quite similar to those of Sxl RBD1 and RBD2, respectively. The individual RBDs of HuC, RBD1 and RBD2 in isolation can interact rather weakly with the minimal ARE motif, AUUUA, while the didomain fragment, RBD1-RBD2, of HuC binds more tightly to a longer ARE RNA, UAUUUAUUUU. Chemical shift perturbations by the longer RNA on HuC RBD1-RBD2 were mapped on and around the two beta-sheets and on the C-terminal region of RBD1. The HuC RBD1-RBD2 residues that exhibited significant chemical shift perturbations coincide with those conserved in Sxl RBD1-RBD2. These data indicate that the RNA-binding characteristics of the HuC and Sxl didomain fragments are similar, even though the target RNAs and the biological functions of the proteins are different.