In ClpXP and ClpAP complexes, ClpA and ClpX use the energy of ATP hydrolysis to unfold proteins and translocate them into the self-compartmentalized ClpP protease. ClpP requires the ATPases to degrade folded or unfolded substrates, but binding of acyldepsipeptide antibiotics (ADEPs) to ClpP bypasses this requirement with unfolded proteins. We present the crystal structure of Escherichia coli ClpP bound to ADEP1 and report the structural changes underlying ClpP activation. ADEP1 binds in the hydrophobic groove that serves as the primary docking site for ClpP ATPases. Binding of ADEP1 locks the N-terminal loops of ClpP in a β-hairpin conformation, generating a stable pore through which extended polypeptides can be threaded. This structure serves as a model for ClpP in the holo-enzyme ClpAP and ClpXP complexes and provides critical information to further develop this class of antibiotics.

Background: Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels control electrical activity through tetramerization of an intracellular linker. Results: NMR shows that the apo-cAMP-binding domain (CBD) of HCN4 destabilizes the tetramer through steric clashes. Conclusion:The apo-HCN4 CBD structure is compatible with monomeric and dimeric but not with tetrameric HCN4. Significance: The proposed mechanism explains HCN auto-inhibition and its relaxation by cAMP.

Effective chromosome organization is central to the functioning of any cell. In bacteria, this organization is achieved through the concerted activity of multiple nucleoid-associated proteins. These proteins are not, however, universally conserved, and different groups of bacteria have distinct subsets that contribute to chromosome architecture. Here, we describe the characterization of a novel actinobacterial-specific protein in Streptomyces coelicolor. We show that sIHF (SCO1480) associates with the nucleoid and makes important contributions to chromosome condensation and chromosome segregation during Streptomyces sporulation. It also affects antibiotic production, suggesting an additional role in gene regulation. In vitro, sIHF binds DNA in a length-dependent but sequence-independent manner, without any obvious structural preferences. It does, however, impact the activity of topoisomerase, significantly altering DNA topology. The sIHF–DNA co-crystal structure reveals sIHF to be composed of two domains: a long N-terminal helix and a C-terminal helix-two turns-helix domain with two separate DNA interaction sites, suggesting a potential role in bridging DNA molecules.

Human serum albumin (HSA) serves not only as a physiological oncotic pressure regulator and a ligand carrier but also as a biomarker for pathologies ranging from ischemia to diabetes. Moreover, HSA is a biopharmaceutical with a growing repertoire of putative clinical applications from hypovolemia to Alzheimer's disease. A key determinant of the physiological, diagnostic, and therapeutic functions of HSA is the amount of long chain fatty acids (LCFAs) bound to HSA. Here, we propose to utilize (13)C-oleic acid for the NMR-based assessment of albumin-bound LCFA concentration (CONFA). (13)C-Oleic acid primes HSA for a LCFA-dependent allosteric transition that modulates the frequency separation between the two main (13)C NMR peaks of HSA-bound oleic acid (ΔνAB). On the basis of ΔνAB, the overall [(12)C-LCFA]Tot/[HSA]Tot ratio is reproducibly estimated in a manner that is only minimally sensitive to glycation, albumin concentration, or redox potential, unlike other methods to quantify HSA-bound LCFAs such as the albumin-cobalt binding assay.

Max-E47 is a protein chimera generated from the fusion of the DNA-binding basic region of Max and the dimerization region of E47, both members of the basic region/helix-loop-helix (bHLH) superfamily of transcription factors. Like native Max, Max-E47 binds with high affinity and specificity to the E-box site, 5′-CACGTG, both in vivo and in vitro. We have determined the crystal structure of Max-E47 at 1.7 Å resolution, and found that it associates to form a well-structured dimer even in the absence of its cognate DNA. Analytical ultracentrifugation confirms that Max-E47 is dimeric even at low micromolar concentrations, indicating that the Max-E47 dimer is stable in the absence of DNA. Circular dichroism analysis demonstrates that both non-specific DNA and the E-box site induce similar levels of helical secondary structure in Max-E47. These results suggest that Max-E47 may bind to the E-box following the two-step mechanism proposed for other bHLH proteins. In this mechanism, a rapid step where protein binds to DNA without sequence specificity is followed by a slow step where specific protein:DNA interactions are fine-tuned, leading to sequence-specific recognition. Collectively, these results show that the designed Max-E47 protein chimera behaves both structurally and functionally like its native counterparts.