Tropical Atlantic multidecadal variability is dominated by external forcing.

The tropical Atlantic climate is characterized by prominent and correlated multidecadal variability in Atlantic sea surface temperatures (SSTs), Sahel rainfall and hurricane activity1-4. Owing to uncertainties in both the models and the observations, the origin of the physical relationships among these systems has remained controversial3-7. Here we show that the cross-equatorial gradient in tropical Atlantic SSTs-largely driven by radiative perturbations associated with anthropogenic emissions and volcanic aerosols since 19503,7-is a key determinant of Atlantic hurricane formation and Sahel rainfall. The relationship is obscured in a large ensemble of CMIP6 Earth system models, because the models overestimate long-term trends for warming in the Northern Hemisphere relative to the Southern Hemisphere from around 1950 as well as associated changes in atmospheric circulation and rainfall. When the overestimated trends are removed, correlations between SSTs and Atlantic hurricane formation and Sahel rainfall emerge as a response to radiative forcing, especially since 1950 when anthropogenic aerosol forcing has been high. Our findings establish that the tropical Atlantic SST gradient is a stronger determinant of tropical impacts than SSTs across the entire North Atlantic, because the gradient is more physically connected to tropical impacts via local atmospheric circulations8. Our findings highlight that Atlantic hurricane activity and Sahel rainfall variations can be predicted from radiative forcing driven by anthropogenic emissions and volcanism, but firmer predictions are limited by the signal-to-noise paradox9-11 and uncertainty in future climate forcings.

Selective and Cell-Active PBRM1 Bromodomain Inhibitors Discovered through NMR Fragment Screening.

PBRM1 is a subunit of the PBAF chromatin remodeling complex that uniquely contains six bromodomains. PBRM1 can operate as a tumor suppressor or tumor promoter. PBRM1 is a tumor promoter in prostate cancer, contributing to migratory and immunosuppressive phenotypes. Selective chemical probes targeting PBRM1 bromodomains are desired to elucidate the association between aberrant PBRM1 chromatin binding and cancer pathogenesis and the contributions of PBRM1 to immunotherapy. Previous PBRM1 inhibitors unselectively bind SMARCA2 and SMARCA4 bromodomains with nanomolar potency. We used our protein-detected NMR screening pipeline to screen 1968 fragments against the second PBRM1 bromodomain, identifying 17 hits with Kd values from 45 µM to >2 mM. Structure-activity relationship studies on the tightest-binding hit resulted in nanomolar inhibitors with selectivity for PBRM1 over SMARCA2 and SMARCA4. These chemical probes inhibit the association of full-length PBRM1 to acetylated histone peptides and selectively inhibit growth of a PBRM1-dependent prostate cancer cell line.

Structural Insights into Molecular Recognition by Human Chemokine CCL19.

The human chemokines CCL19 and CCL21 bind to the G protein-coupled receptor (GPCR) CCR7 and play an important role in the trafficking of immune cells as well as cancer metastasis. Conserved binding sites for sulfotyrosine residues on the receptor contribute significantly to the chemokine/GPCR interaction and have been shown to provide promising targets for new drug-discovery efforts to disrupt the chemokine/GPCR interaction and, consequently, tumor metastasis. Here, we report the first X-ray crystal structure of a truncated CCL19 (residues 7-70) at 2.50 Å resolution, revealing molecular details crucial for protein-protein interactions. Although the overall structure is similar to the previously determined NMR model, there are important variations, particularly near the N terminus and the so-called 30's and 40's loops. Computational analysis using the FTMap server indicates the potential importance of these areas in ligand binding and the differences in binding hotspots compared to CCL21. NMR titration experiments using a CCR7-derived peptide (residues 5-11, TDDYIGD) further demonstrate potential receptor recognition sites, such as those near the C terminus and 40's loop, which consist of both positively charged and hydrophobic residues that may be important for receptor binding. Taken together, the X-ray, NMR, and computational analysis herein provide insights into the overall structure and molecular features of CCL19 and enables investigation into this chemokine's function and inhibitor development.

Trisubstituted 1,3,5-Triazines: The First Ligands of the sY12-Binding Pocket on Chemokine CXCL12.

CXCL12, a CXC-type chemokine, binds its receptor CXCR4, and the resulting signaling cascade is essential during development and subsequently in immune function. Pathologically, the CXCL12-CXCR4 signaling axis is involved in many cancers and inflammatory diseases and thus has sparked continued interest in the development of therapeutics. Small molecules targeting CXCR4 have had mixed results in clinical trials. Alternatively, small molecules targeting the chemokine instead of the receptor provide a largely unexplored space for therapeutic development. Here we report that trisubstituted 1,3,5-triazines are competent ligands for the sY12-binding pocket of CXCL12. The initial hit was optimized to be more synthetically tractable. Fifty unique triazines were synthesized, and the structure-activity relationship was probed. Using computational modeling, we suggest key structural interactions that are responsible for ligand-chemokine binding. The lipophilic ligand efficiency was improved, resulting in more soluble, drug-like molecules with chemical handles for future development and structural studies.

Streptomyces wadayamensis MppP is a PLP-Dependent Oxidase, Not an Oxygenase.

The PLP-dependent l-arginine hydroxylase/deaminase MppP from Streptomyces wadayamensis (SwMppP) is involved in the biosynthesis of l-enduracididine, a nonproteinogenic amino acid found in several nonribosomally produced peptide antibiotics. SwMppP uses only PLP and molecular oxygen to catalyze a 4-electron oxidation of l-arginine to form a mixture of 2-oxo-4(S)-hydroxy-5-guanidinovaleric acid and 2-oxo-5-guanidinovaleric acid. Steady-state kinetics analysis in the presence and absence of catalase shows that one molecule of peroxide is formed for every molecule of dioxygen consumed in the reaction. Moreover, for each molecule of 2-oxo-4(S)-hydroxy-5-guanidinovaleric acid produced, two molecules of dioxygen are consumed, suggesting that both the 4-hydroxy and 2-keto groups are derived from water. This was confirmed by running the reactions using either [18]O2 or H2[18]O and analyzing the products by ESI-MS. Incorporation of [18]O was only observed when the reaction was performed in H2[18]O. Crystal structures of SwMppP with l-arginine, 2-oxo-4(S)-hydroxy-5-guanidinovaleric acid, or 2-oxo-5-guanidinovaleric acid bound were determined at resolutions of 2.2, 1.9. and 1.8 Å, respectively. The structural data show that the N-terminal portion of the protein is disordered unless substrate or product is bound in the active site, in which case it forms a well-ordered helix that covers the catalytic center. This observation suggested that the N-terminal helix may have a role in substrate binding and/or catalysis. Our structural and kinetic characterizations of N-terminal variants show that the N-terminus is critical for catalysis. In light of this new information, we have refined our previously proposed mechanism of the SwMppP-catalyzed oxidation of l-arginine.

Swit_4259, an acetoacetate decarboxylase-like enzyme from Sphingomonas wittichii RW1.

The Gram-negative bacterium Sphingomonas wittichii RW1 is notable for its ability to metabolize a variety of aromatic hydrocarbons. Not surprisingly, the S. wittichii genome contains a number of putative aromatic hydrocarbon-degrading gene clusters. One of these includes an enzyme of unknown function, Swit_4259, which belongs to the acetoacetate decarboxylase-like superfamily (ADCSF). Here, it is reported that Swit_4259 is a small (28.8 kDa) tetrameric ADCSF enzyme that, unlike the prototypical members of the superfamily, does not have acetoacetate decarboxylase activity. Structural characterization shows that the tertiary structure of Swit_4259 is nearly identical to that of the true decarboxylases, but there are important differences in the fine structure of the Swit_4259 active site that lead to a divergence in function. In addition, it is shown that while it is a poor substrate, Swit_4259 can catalyze the hydration of 2-oxo-hex-3-enedioate to yield 2-oxo-4-hydroxyhexanedioate. It is also demonstrated that Swit_4259 has pyruvate aldolase-dehydratase activity, a feature that is common to all of the family V ADCSF enzymes studied to date. The enzymatic activity, together with the genomic context, suggests that Swit_4259 may be a hydratase with a role in the metabolism of an as-yet-unknown hydrocarbon. These data have implications for engineering bioremediation pathways to degrade specific pollutants, as well as structure-function relationships within the ADCSF in general.