Structural insights into IMP2 dimerization and RNA binding.

IGF2BP2 (IMP2) is an RNA-binding protein that contributes to cancer tumorigenesis and metabolic disorders. Structural studies focused on individual IMP2 domains have provided important mechanistic insights into IMP2 function; however, structural information on full-length IMP2 is lacking but necessary to understand how to target IMP2 activity in drug discovery. In this study, we investigated the behavior of full-length IMP2 and the influence of RNA binding using biophysical and structural methods including mass photometry, hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS), and small angle x-ray scattering (SAXS). We found that full-length IMP2 forms multiple oligomeric states but predominantly adopts a dimeric conformation. Molecular models derived from SAXS data suggest the dimer is formed in a head-to-tail orientation by the KH34 and RRM1 domains. Upon RNA binding, IMP2 forms a pseudo-symmetric dimer different from its apo/RNA-free state, with the KH12 domains of each IMP2 molecule forming the dimer interface. We also found that the formation of IMP2 oligomeric species, which includes dimers and higher-order oligomers, is sensitive to ionic strength and RNA binding. Our findings provide the first insight into the structural properties of full-length IMP2, which may lead to novel opportunities for disrupting its function with more effective IMP2 inhibitors.

Orphan receptor GPR158 serves as a metabotropic glycine receptor: mGlyR.

Glycine is a major neurotransmitter involved in several fundamental neuronal processes. The identity of the metabotropic receptor mediating slow neuromodulatory effects of glycine is unknown. We identified an orphan G protein-coupled receptor, GPR158, as a metabotropic glycine receptor (mGlyR). Glycine and a related modulator, taurine, directly bind to a Cache domain of GPR158, and this event inhibits the activity of the intracellular signaling complex regulator of G protein signaling 7-G protein ß5 (RGS7-Gß5), which is associated with the receptor. Glycine signals through mGlyR to inhibit production of the second messenger adenosine 3',5'-monophosphate. We further show that glycine, but not taurine, acts through mGlyR to regulate neuronal excitability in cortical neurons. These results identify a major neuromodulatory system involved in mediating metabotropic effects of glycine, with implications for understanding cognition and affective states.

Sculpting a Uniquely Reactive Cysteine Residue for Site-Specific Antibody Conjugation.

Catalytic antibody 38C2 and its humanized version h38C2 harbor a uniquely reactive lysine at the bottom of a 11 Å deep pocket that permits site-specific conjugation of ß-diketone-, ß-lactam-, and heteroaryl methylsulfonyl-functionalized small and large molecules. Various dual variable domain formats pair a tumor-targeting antibody with h38C2 to enable precise, fast, and stable assembly of antibody-drug conjugates (ADCs). Here, we expand the scope of this ADC assembly strategy by mutating h38C2's reactive lysine to a cysteine. X-ray crystallography of this point mutant, h38C2_K99C, confirmed a deeply buried unpaired cysteine. Probing h38C2_K99C with maleimide, monobromomaleimide, and dibromomaleimide derivatives of a fluorophore revealed highly disparate conjugation efficiencies and stabilities. Dibromomaleimide emerged as a suitable electrophile for the precise, fast, efficient, and stable assembly of ADCs with the h38C2_K99C module. Mass spectrometry indicated the presence of a thio-monobromomaleimide linkage which was further supported by in silico docking studies. Using a dibromomaleimide derivative of the highly potent tubulin polymerization inhibitor monomethyl auristatin F, h38C2_K99C-based ADCs were found to be as potent as h38C2-based ADCs and afford a new assembly route for ADCs with single and dual payloads.

Structural plasticity of the selectivity filter in a nonselective ion channel.

The sodium potassium ion channel (NaK) is a nonselective ion channel that conducts both sodium and potassium across the cellular membrane. A new crystallographic structure of NaK reveals conformational differences in the residues that make up the selectivity filter between the four subunits that form the ion channel and the inner helix of the ion channel. The crystallographic structure also identifies a side-entry, ion-conduction pathway for Na+ permeation that is unique to NaK. NMR studies and molecular dynamics simulations confirmed the dynamical nature of the top part of the selectivity filter and the inner helix in NaK as also observed in the crystal structure. Taken together, these results indicate that the structural plasticity of the selectivity filter combined with the dynamics of the inner helix of NaK are vital for the efficient conduction of different ions through the non-selective ion channel of NaK.

Molecular dynamics analysis of Mg2+ -dependent cleavage of a pistol ribozyme reveals a fail-safe secondary ion for catalysis.

Pistol ribozymes comprise a class of small, self-cleaving RNAs discovered via comparative genomic analysis. Prior work in the field has probed the kinetics of the cleavage reaction, as well as the influence of various metal ion cofactors that accelerate the process. In the current study, we performed unbiased and unconstrained molecular dynamics simulations from two current high-resolution pistol crystal structures, and we analyzed trajectory data within the context of the currently accepted ribozyme mechanistic framework. Root-mean-squared deviations, radial distribution functions, and distributions of nucleophilic angle-of-attack reveal insights into the potential roles of three magnesium ions with respect to catalysis and overall conformational stability of the molecule. A series of simulation trajectories containing in silico mutations reveal the relatively flexible and partially interchangeable roles of two particular magnesium ions within solvated hydrogen-bonding distances from the catalytic center.

Structures of proline-rich peptides bound to the ribosome reveal a common mechanism of protein synthesis inhibition.

With bacterial resistance becoming a serious threat to global public health, antimicrobial peptides (AMPs) have become a promising area of focus in antibiotic research. AMPs are derived from a diverse range of species, from prokaryotes to humans, with a mechanism of action that often involves disruption of the bacterial cell membrane. Proline-rich antimicrobial peptides (PrAMPs) are instead actively transported inside the bacterial cell where they bind and inactivate specific targets. Recently, it was reported that some PrAMPs, such as Bac71 -35, oncocins and apidaecins, bind and inactivate the bacterial ribosome. Here we report the crystal structures of Bac71 -35, Pyrrhocoricin, Metalnikowin and two oncocin derivatives, bound to the Thermus thermophilus 70S ribosome. Each of the PrAMPs blocks the peptide exit tunnel of the ribosome by simultaneously occupying three well characterized antibiotic-binding sites and interferes with the initiation step of translation, thereby revealing a common mechanism of action used by these PrAMPs to inactivate protein synthesis. Our study expands the repertoire of PrAMPs and provides a framework for designing new-generation therapeutics.

The mechanism of inhibition of protein synthesis by the proline-rich peptide oncocin.

Antibiotic-resistant bacteria are a global health issue necessitating the development of new effective therapeutics. Proline-rich antimicrobial peptides (PrAMPs), which include oncocins, are an extensively studied class of AMPs that counteract bacterial infection at submicromolar concentrations. Oncocins enter and kill bacteria by inhibiting certain targets rather than by acting through membrane lysis. Although they have recently been reported to bind DnaK and the bacterial ribosome, their mode of inhibition has remained elusive. Here we report the crystal structure of the oncocin derivative Onc112 bound to the Thermus thermophilus 70S ribosome. Strikingly, this 19-residue proline-rich peptide manifests the features of several known classes of ribosome inhibitors by simultaneously blocking the peptidyl transferase center and the peptide-exit tunnel of the ribosome. This high-resolution structure thus reveals the mechanism by which oncocins inhibit protein synthesis, providing an opportunity for structure-based design of new-generation therapeutics.