Interactions between the quality control ubiquitin ligase CHIP and ubiquitin conjugating enzymes

<p>Abstract</p> <p>Background</p> <p>Ubiquitin (E3) ligases interact with specific ubiquitin conjugating (E2) enzymes to ubiquitinate particular substrate proteins. As the combination of E2 and E3 dictates the type and biological consequence of ubiquitination, it is important to understand the basis of specificity in E2:E3 interactions. The E3 ligase CHIP interacts with Hsp70 and Hsp90 and ubiquitinates client proteins that are chaperoned by these heat shock proteins. CHIP interacts with two types of E2 enzymes, UbcH5 and Ubc13-Uev1a. It is unclear, however, why CHIP binds these E2 enzymes rather than others, and whether CHIP interacts preferentially with UbcH5 or Ubc13-Uev1a, which form different types of polyubiquitin chains.</p> <p>Results</p> <p>The 2.9 Å crystal structure of the CHIP U-box domain complexed with UbcH5a shows that CHIP binds to UbcH5 and Ubc13 through similar specificity determinants, including a key S-P-A motif on the E2 enzymes. The determinants make different relative contributions to the overall interactions between CHIP and the two E2 enzymes. CHIP undergoes auto-ubiquitination by UbcH5 but not by Ubc13-Uev1a. Instead, CHIP drives the formation of unanchored polyubiquitin by Ubc13-Uev1a. CHIP also interacts productively with the class III E2 enzyme Ube2e2, in which the UbcH5- and Ubc13-binding specificity determinants are highly conserved.</p> <p>Conclusion</p> <p>The CHIP:UbcH5a structure emphasizes the importance of specificity determinants located on the long loops and central helix of the CHIP U-box, and on the N-terminal helix and loops L4 and L7 of its cognate E2 enzymes. The S-P-A motif and other specificity determinants define the set of cognate E2 enzymes for CHIP, which likely includes several Class III E2 enzymes. CHIP's interactions with UbcH5, Ube2e2 and Ubc13-Uev1a are consistent with the notion that Ubc13-Uev1a may work sequentially with other E2 enzymes to carry out K63-linked polyubiquitination of CHIP substrates.</p

YbtT is a low-specificity type II thioesterase that maintains production of the metallophore yersiniabactin in pathogenic enterobacteria

Clinical isolates of Yersinia, Klebsiella, and Escherichia coli frequently secrete the small molecule metallophore yersiniabactin (Ybt), which passivates and scavenges transition metals during human infections. YbtT is encoded within the Ybt biosynthetic operon and is critical for full Ybt production in bacteria. However, its biosynthetic function has been unclear because it is not essential for Ybt production by the in vitro reconstituted nonribosomal peptide synthetase/polyketide synthase (NRPS/PKS) pathway. Here, we report the structural and biochemical characterization of YbtT. YbtT structures at 1.4-1.9 Å resolution possess a serine hydrolase catalytic triad and an associated substrate chamber with features similar to those previously reported for low-specificity type II thioesterases (TEIIs). We found that YbtT interacts with the two major Ybt biosynthetic proteins, HMWP1 (high-molecular-weight protein 1) and HMWP2 (high-molecular-weight protein 2), and hydrolyzes a variety of aromatic and acyl groups from their phosphopantetheinylated carrier protein domains. In vivo YbtT titration in uropathogenic E. coli revealed a distinct optimum for Ybt production consistent with a tradeoff between clearing both stalled inhibitory intermediates and productive Ybt precursors from HMWP1 and HMWP2. These results are consistent with a model in which YbtT maintains cellular Ybt biosynthesis by removing nonproductive, inhibitory thioesters that form aberrantly at multiple sites on HMWP1 and HMWP2

Structural basis of nucleotide exchange and client binding by the Hsp70 cochaperone Bag2

Cochaperones are essential for Hsp70/Hsc70-mediated folding of proteins and include nucleotide exchange factors (NEF) that assist protein folding by accelerating ADP/ATP exchange on Hsp70. The cochaperone Bag2 binds misfolded Hsp70 clients and also acts as a NEF, but the molecular basis of its functions is unclear. We show that, rather than being a member of the Bag domain family, Bag2 contains a new type of Hsp70 NEF domain, which we call the “Brand New Bag” (BNB) domain. Free and Hsc70-bound crystal structures of Bag2-BNB show its dimeric structure in which a flanking linker helix and loop bind to Hsc70 to promote nucleotide exchange. NMR analysis demonstrates that the client-binding sites and Hsc70 interaction sites of Bag2-BNB overlap, and that Hsc70 can displace clients from Bag2-BNB, indicating a distinct mechanism for the regulation of Hsp-70-mediated protein folding by Bag2

Coordination of di-acetylated histone ligands by the ATAD2 bromodomain

Open access article. Creative Commons Attribution 4.0 International license (CC BY 4.0) appliesThe ATPase Family, AAA domain-containing protein 2 (ATAD2) bromodomain (BRD) has a canonical bromodomain structure consisting of four -helices. ATAD2 functions as a coactivator of the androgen and estrogen receptors as well as the MYC and E2F transcription factors. ATAD2 also functions during DNA replication, recognizing newly synthesized histones. In addition, ATAD2 is shown to be up-regulated in multiple forms of cancer including breast, lung, gastric, endometrial, renal, and prostate. Furthermore, up-regulation of ATAD2 is strongly correlated with poor prognosis in many types of cancer, making the ATAD2 bromodomain an innovative target for cancer therapeutics. In this study, we describe the recognition of histone acetyllysine modifications by the ATAD2 bromodomain. Residue-specific information on the complex formed between the histone tail and the ATAD2 bromodomain, obtained through nuclear magnetic resonance spectroscopy (NMR) and X-ray crystallography, illustrates key residues lining the binding pocket, which are involved in coordination of di-acetylated histone tails. Analytical ultracentrifugation, NMR relaxation data, and isothermal titration calorimetry further confirm the monomeric state of the functionally active ATAD2 bromodomain in complex with di-acetylated histone ligands. Overall, we describe histone tail recognition by ATAD2 BRD and illustrate that one acetyllysine group is primarily engaged by the conserved asparagine (N1064), the “RVF” shelf residues, and the flexible ZA loop. Coordination of a second acetyllysine group also occurs within the same binding pocket but is essentially governed by unique hydrophobic and electrostatic interactions making the di-acetyllysine histone coordination more specific than previously presumed.Ye

Structural analysis and DNA binding of the HMG domains of the human mitochondrial transcription factor A

The mitochondrial transcription factor A (mtTFA) is central to assembly and initiation of the mitochondrial transcription complex. Human mtTFA (h-mtTFA) is a dual high mobility group box (HMGB) protein that binds site-specifically to the mitochondrial genome and demarcates the promoters for recruitment of h-mtTFB1, h-mtTFB2 and the mitochondrial RNA polymerase. The stoichiometry of h-mtTFA was found to be a monomer in the absence of DNA, whereas it formed a dimer in the complex with the light strand promoter (LSP) DNA. Each of the HMG boxes and the C-terminal tail were evaluated for their ability to bind to the LSP DNA. Removal of the C-terminal tail only slightly decreased nonsequence specific DNA binding, and box A, but not box B, was capable of binding to the LSP DNA. The X-ray crystal structure of h-mtTFA box B, at 1.35 Å resolution, revealed the features of a noncanonical HMG box. Interactions of box B with other regions of h-mtTFA were observed. Together, these results provide an explanation for the unusual DNA-binding properties of box B and suggest possible roles for this domain in transcription complex assembly

Evaluation of integrin αvβ6 cystine knot PET tracers to detect cancer and idiopathic pulmonary fibrosis.

Advances in precision molecular imaging promise to transform our ability to detect, diagnose and treat disease. Here, we describe the engineering and validation of a new cystine knot peptide (knottin) that selectively recognizes human integrin αvβ6 with single-digit nanomolar affinity. We solve its 3D structure by NMR and x-ray crystallography and validate leads with 3 different radiolabels in pre-clinical models of cancer. We evaluate the lead tracer's safety, biodistribution and pharmacokinetics in healthy human volunteers, and show its ability to detect multiple cancers (pancreatic, cervical and lung) in patients at two study locations. Additionally, we demonstrate that the knottin PET tracers can also detect fibrotic lung disease in idiopathic pulmonary fibrosis patients. Our results indicate that these cystine knot PET tracers may have potential utility in multiple disease states that are associated with upregulation of integrin αvβ6

Individual Ion Binding Sites in the K(+) Channel Play Distinct Roles in C-type Inactivation and in Recovery from Inactivation.

The selectivity filter of K(+) channels contains four ion binding sites (S1-S4) and serves dual functions of discriminating K(+) from Na(+) and acting as a gate during C-type inactivation. C-type inactivation is modulated by ion binding to the selectivity filter sites, but the underlying mechanism is not known. Here we evaluate how the ion binding sites in the selectivity filter of the KcsA channel participate in C-type inactivation and in recovery from inactivation. We use unnatural amide-to-ester substitutions in the protein backbone to manipulate the S1-S3 sites and a side-chain substitution to perturb the S4 site. We develop an improved semisynthetic approach for generating these amide-to-ester substitutions in the selectivity filter. Our combined electrophysiological and X-ray crystallographic analysis of the selectivity filter mutants show that the ion binding sites play specific roles during inactivation and provide insights into the structural changes at the selectivity filter during C-type inactivation

Individual Ion Binding Sites in the K(+) Channel Play Distinct Roles in C-type Inactivation and in Recovery from Inactivation.

The selectivity filter of K(+) channels contains four ion binding sites (S1-S4) and serves dual functions of discriminating K(+) from Na(+) and acting as a gate during C-type inactivation. C-type inactivation is modulated by ion binding to the selectivity filter sites, but the underlying mechanism is not known. Here we evaluate how the ion binding sites in the selectivity filter of the KcsA channel participate in C-type inactivation and in recovery from inactivation. We use unnatural amide-to-ester substitutions in the protein backbone to manipulate the S1-S3 sites and a side-chain substitution to perturb the S4 site. We develop an improved semisynthetic approach for generating these amide-to-ester substitutions in the selectivity filter. Our combined electrophysiological and X-ray crystallographic analysis of the selectivity filter mutants show that the ion binding sites play specific roles during inactivation and provide insights into the structural changes at the selectivity filter during C-type inactivation