Structural basis for cell type specific DNA binding of C/EBPß: The case of cell cycle inhibitor p15INK4b promoter.

C/EBPß is a key regulator of numerous cellular processes, but it can also contribute to tumorigenesis and viral diseases. It binds to specific DNA sequences (C/EBP sites) and interacts with other transcription factors to control expression of multiple eukaryotic genes in a tissue and cell-type dependent manner. A body of evidence has established that cell-type-specific regulatory information is contained in the local DNA sequence of the binding motif. In human epithelial cells, C/EBPß is an essential cofactor for TGFß signaling in the case of Smad2/3/4 and FoxO-dependent induction of the cell cycle inhibitor, p15INK4b. In the TGFß-responsive region 2 of the p15INK4b promoter, the Smad binding site is flanked by a C/EBP site, CTTAA•GAAAG, which differs from the canonical, palindromic ATTGC•GCAAT motif. The X-ray crystal structure of C/EBPß bound to the p15INK4b promoter fragment shows how GCGC-to-AAGA substitution generates changes in the intermolecular interactions in the protein-DNA interface that enhances C/EBPß binding specificity, limits possible epigenetic regulation of the promoter, and generates a DNA element with a unique pattern of methyl groups in the major groove. Significantly, CT/GA dinucleotides located at the 5'ends of the double stranded element maintain local narrowing of the DNA minor groove width that is necessary for DNA recognition. Our results suggest that C/EBPß would accept all forms of modified cytosine in the context of the CpT site. This contrasts with the effect on the consensus motif, where C/EBPß binding is modestly increased by cytosine methylation, but substantially decreased by hydroxymethylation.

DASH diet vs. DASH diet plus physical activity in older patients with type 2 diabetes and high blood pressure: A randomized clinical trial.

BACKGROUND AND AIMS: To evaluate the effect of lifestyle modification by adopting a DASH diet, with and without physical activity guidance, on blood pressure, glycemic control, lipid profile, weight, and body composition in older patients with type 2 diabetes mellitus (T2DM) and hypertension. METHODS AND RESULTS: For this randomized clinical trial, we recruited patients aged 60 years or older with T2DM and uncontrolled hypertension. One group (DASH) received only DASH dietary guidance, while the other group (DASHPED) received dietary guidance and encouragement to walk with a pedometer. Outcomes of interest were (1) blood pressure, (2) physical activity, (3) weight, body mass index (BMI), and body composition, and (4) biochemical variables. Measurements were taken at baseline and 16 weeks after the intervention. We included 35 patients in the analysis. At the end of the study, the DASHPED group had an mean increase in physical activity of 1721 steps/day. Both groups displayed significantly reduced weight, BMI, and waking diastolic pressures on ambulatory blood pressure monitoring after the intervention. A trend of reduced sleeping diastolic pressure was found in the DASHPED group. Changes in weight, BMI, muscle mass, body fat, waist-hip ratio, glycemic control, lipid profile, and insulin sensitivity did not differ between the groups. CONCLUSION: There was no difference in outcomes between the group that only dieted and the group that also performed increased physical activity, despite a significant increase in exercise. This reinforces the importance of dietary changes in immediate blood pressure control.

Protein Adsorption and Layer Formation at the Stainless Steel-Solution Interface Mediates Shear-Induced Particle Formation for an IgG1 Monoclonal Antibody.

Passage of specific protein solutions through certain pumps, tubing, and/or filling nozzles can result in the production of unwanted subvisible protein particles (SVPs). In this work, surface-mediated SVP formation was investigated. Specifically, the effects of different solid interface materials, interfacial shear rates, and protein concentrations on SVP formation were measured for the National Institute of Standards and Technology monoclonal antibody (NISTmAb), a reference IgG1 monoclonal antibody (mAb). A stainless steel rotary piston pump was used to identify formulation and process parameters that affect aggregation, and a flow cell (alumina or stainless steel interface) was used to further investigate the effect of different interface materials and/or interfacial shear rates. SVP particles produced were monitored using flow microscopy or flow cytometry. Neutron reflectometry and a quartz crystal microbalance with dissipation monitoring were used to characterize adsorption and properties of NISTmAb at the stainless steel interface. Pump/shear cell experiments showed that the NISTmAb concentration and interface material had a significant effect on SVP formation, while the effects of interfacial shear rate and passage number were less important. At the higher NISTmAb concentrations, the adsorbed protein became structurally altered at the stainless steel interface. The primary adsorbed layer remained largely undisturbed during flow, suggesting that SVP formation at high NISTmAb concentration was caused by the disruption of patches and/or secondary interactions.

Interactions of CCAAT/enhancer-binding protein ß with transcriptional coregulators.

CCAAT/enhancer-binding proteins (C/EBPs) are key regulators of numerous cellular processes, including cell proliferation, differentiation, and tumorigenesis. Their biological activities require interactions with several protein partners and the formation of functional multiprotein complexes involved in DNA repair and cell cycle control. Members of the family (C/EBPα, ß, δ, ε, and γ) bind to common DNA sequence motifs as homo- or hetero-dimers and interact with other transcription factors to control transcription of a number of eukaryotic genes. Of particular interest is C/EBPß, which binds to closed chromatin and acts as a pioneering factor for initiating tissue-specific gene expression at several promoters. This review focuses on intermolecular interactions that underlie C/EBPß's ability to regulate chromatin accessibility and directs readers to general reviews describing transcription regulation in eukaryotes.

Structural insights into interactions of C/EBP transcriptional activators with the Taz2 domain of p300.

Members of the C/EBP family of transcription factors bind to the Taz2 domain of p300/CBP and mediate its phosphorylation through the recruitment of specific kinases. Short sequence motifs termed homology boxes A and B, which comprise their minimal transactivation domains (TADs), are conserved between C/EBP activators and are necessary for specific p300/CBP binding. A possible mode of interaction between C/EBP TADs and the p300 Taz2 domain was implied by the crystal structure of a chimeric protein composed of residues 1723-1818 of p300 Taz2 and residues 37-61 of C/EBPℇ. The segment corresponding to the C/EBPℇ TAD forms two orthogonally disposed helices connected by a short linker and interacts with the core structure of Taz2 from a symmetry-related molecule. It is proposed that other members of the C/EBP family interact with the Taz2 domain in the same manner. The position of the C/EBPℇ peptide on the Taz2 protein interaction surface suggests that the N-termini of C/EBP proteins are unbound in the C/EBP-p300 Taz2 complex. This observation is in agreement with the known location of the docking site of protein kinase HIPK2 in the C/EBPß N-terminus, which associates with the C/EBPß-p300 complex.

Rescue therapy with upadacitinib in medically refractory pediatric ulcerative colitis.

Approved options for advanced therapy in pediatric inflammatory bowel disease (IBD) are limited. Although Janus kinase (JAK) inhibitors are approved in adult IBD, their benefit in pediatric populations is not yet delineated. We present a 13-year-old female patient with ulcerative colitis (UC) refractory to numerous therapies and courses of prednisone that ultimately responded to a JAK inhibitor. Initial treatment consisted of 5-aminosalicylate and azathioprine. This was changed to adalimumab due to persistent symptoms. Repeat colonoscopy revealed pancolitis, thus she was transitioned to vedolizumab. She was hospitalized twice for uncontrolled symptoms on vedolizumab and subsequent scope showed continued pancolitis. As a result, she transitioned to ustekinumab without symptomatic relief after adjusting to monthly dosing. The family declined colectomy, opting to exhaust all medical therapies. Upadacitinib was started and her symptoms resolved within 1 week, and she remains in steroid-free remission. This case illustrates the possible role of JAK inhibitors in extensively refractory pediatric UC patients before colectomy.

Crystal structures of the reverse transcriptase-associated ribonuclease H domain of xenotropic murine leukemia-virus related virus.

The ribonuclease H (RNase H) domain of retroviral reverse transcriptase (RT) plays a critical role in the life cycle by degrading the RNA strands of DNA/RNA hybrids. In addition, RNase H activity is required to precisely remove the RNA primers from nascent (-) and (+) strand DNA. We report here three crystal structures of the RNase H domain of xenotropic murine leukemia virus-related virus (XMRV) RT, namely (i) the previously identified construct from which helix C was deleted, (ii) the intact domain, and (iii) the intact domain complexed with an active site α-hydroxytropolone inhibitor. Enzymatic assays showed that the intact RNase H domain retained catalytic activity, whereas the variant lacking helix C was only marginally active, corroborating the importance of this helix for enzymatic activity. Modeling of the enzyme-substrate complex elucidated the essential role of helix C in binding a DNA/RNA hybrid and its likely mode of recognition. The crystal structure of the RNase H domain complexed with ß-thujaplicinol clearly showed that coordination by two divalent cations mediates recognition of the inhibitor.

Geographic distribution of risk of death due to homicide in Puerto Rico, 2001-2010.

OBJECTIVE: To raise awareness of the impact of homicides in Puerto Rico based on the findings of the spatial and temporal distribution of homicides and the use of firearms, by age and gender, using reports of interpersonal violent deaths from the Institute of Forensic Science (IFS) headquartered in San Juan, Puerto Rico. METHODS: This was a descriptive study of all homicide incidents in Puerto Rico reported by the IFS for the period 2001-2010. For each of the 8 542 cases, data analyzed included age, sex, municipality of incident, date of death, and mechanism. Crude sex- and age-specific mortality rates for Puerto Rico and for each municipality per year and for the 10-year period were calculated. Cumulative rate and cumulative risks were estimated and defined as lifetime risk. The relative distribution of cumulative rates for each municipality was categorized into quartiles of highest to lowest risk and displayed as a map. RESULTS: The risk of homicide death among males is 13 times greater than among females. The highest rates were observed among males 20-24 years of age (198.4 homicides per 100 000). In any given year, firearms were used in at least 80% of homicides. The average lifetime risk of homicide death for males is 1 in 34. CONCLUSIONS: Young adult males with access to firearms are at greatest risk of homicide in Puerto Rico. Also, highly urbanized municipalities are at highest risk; however, certain non-urban municipalities along the coast also have a very high homicide risk. Top priorities should be applying the WHO "ecological model" for violent injury prevention and establishing a surveillance system that will assist in identifying the role that socioeconomics, illegal firearms trade, and drug trafficking are playing.

Flocculated amorphous itraconazole nanoparticles for enhanced in vitro supersaturation and in vivo bioavailability.

Rapid flocculation of nanoparticle dispersions of a poorly water soluble drug, itraconazole (Itz), was utilized to produce amorphous powders with desirable dissolution properties for high bioavailability in rats. Antisolvent precipitation (AP) was utilized to form Itz nanodispersions with high drug loadings stabilized with hydroxypropylmethylcellulose (HPMC) or the pH-sensitive Eudragit(®) L100-55 (EL10055). The HPMC dispersions were flocculated by desolvating the polymer through the addition of a divalent salt, and the enteric EL10055 by reducing the pH. The formation of open flocs by diffusion limited aggregation facilitated redispersion of the flocs at pH 6.8. Upon redispersion of the flocculated nanoparticles at pH 6.8, the particle size was modestly larger than the original size, on the order of 1 µm. High in vitro supersaturation (AUC) of the flocculated nanoparticle dispersions was observed in micellar media at pH 6.8, after 2 hours initial exposure at pH 1.2 to simulate the stomach, relative to the AUC for a commercially available Itz formulation, Sporanox. Greater in vivo bioavailability in rats was correlated directly to the higher in vitro AUC at pH 6.8 with micelles during the pH shift experiment for the flocculated nanoparticle dispersions relative to Sporanox. The ability to generate and sustain high supersaturation in micellar media at pH 6.8, as shown with the in vitro pH shift dissolution test, is beneficial for increasing bioavailability of Itz by oral delivery.

Post-mortem COVID-19 Positive Testing: Institute of Forensic Sciences Experience.

OBJECTIVE: The main objective was to present the experience of the Institute of Forensic Sciences of Puerto Rico in facing the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), also known as COVID-19. It has been found that some COVID-19 positive cases may continue to show post-mortem positive results for up to 49 days. METHODS: The in vitro technique of ID NOW COVID-19 was used in the analysis to evaluate the presence of SARS-Cov-2 in postmortem forensic cases. This isothermal method allows to amplify and identify the presence of the RNA-dependent RNA polymerase viral segment. Information on demographics, comorbidities, and the manner and cause of death was collected. RESULTS: A total of 612 subjects were sampled, of which 41 (6.7%) tested positive for COVID-19;14 (34.1%) of those subjects remained positive for more than 7 days Postmortem. Of the 41 positive cases, only 3 (7.3%) had been diagnosed with COVID-19 before their demise. The most common comorbidities were hypertension (36%), obesity (29%), and mental health conditions (50%). CONCLUSION: Results from postmortem COVID-19 testing revealed that some cadavers remain COVID-19 positive for a longer period than expected. Despite this, based on the information collected from the cases that were tested more than once, there is no direct correlation between the cause of death and persistent COVID-19 positivity. We recommend that additional investigations be carried out, in which investigations viral load and the maximum time of the infectious phase are specifically evaluated.

CCAAT/Enhancer-binding protein beta DNA binding is auto-inhibited by multiple elements that also mediate association with p300/CREB-binding protein (CBP).

Signaling through Ras GTPases controls the activity of many transcription factors including CCAAT/enhancer-binding protein (C/EBPbeta), which regulates oncogenic H-Ras(V12)-induced senescence and growth arrest. Here we report that C/EBPbeta (LAP) DNA binding is inhibited by N-terminal sequences and derepressed by oncogenic Ras signaling. Sequence and mutational analyses showed that auto-repression involves two LXXLF (phiXXphiphi)-like motifs (LX1 and LX2) and a third element, auto-inhibitory domain (AID), located within conserved region CR5. LX1 is a critical component of the transactivation domain and has been shown to mediate C/EBPbeta binding to the TAZ2 region of p300/CREB-binding protein coactivators. C/EBPbeta auto-repression also involves a C-terminal regulatory domain (CRD) adjacent to the leucine zipper. CRD contains a third phiXXphiphi motif (LX3) and a short sequence, KQL, which has similarity to a region in the protein-binding site of TAZ2. The C/EBPbeta N- and C-terminal domains physically associate in a manner that requires the basic region and CRD. We propose a model in which the regulatory sequences form a hydrophobic core that reciprocally inhibits DNA binding and transactivation. We also suggest a mechanism for C/EBPbeta derepression involving several recently identified modifications within AID and CRD. Finally, we show that association of activated C/EBPbeta with p300/CREB-binding protein requires the LX2 and AID auto-inhibitory elements. Thus, the N-terminal regulatory elements have dual roles in auto-inhibition and coactivator binding.

Structural basis for p300 Taz2-p53 TAD1 binding and modulation by phosphorylation.

Coactivators CREB-binding protein and p300 play important roles in mediating the transcriptional activity of p53. Until now, however, no detailed structural information has been available on how any of the domains of p300 interact with p53. Here, we report the NMR structure of the complex of the Taz2 (C/H3) domain of p300 and the N-terminal transactivation domain of p53. In the complex, p53 forms a short alpha helix and interacts with the Taz2 domain through an extended surface. Mutational analyses demonstrate the importance of hydrophobic residues for complex stabilization. Additionally, they suggest that the increased affinity of Taz2 for p53(1-39) phosphorylated at Thr(18) is due in part to electrostatic interactions of the phosphate with neighboring arginine residues in Taz2. Thermodynamic experiments revealed the importance of hydrophobic interactions in the complex of Taz2 with p53 phosphorylated at Ser(15) and Thr(18).

Automated TTF-1 Immunohistochemistry Assay for the Differentiation of Lung Adenocarcinoma Versus Lung Squamous Cell Carcinoma.

Due to therapeutic advances, the subclassification of non-small cell lung carcinomas (NSCLC) between the adenocarcinomas and squamous cell carcinomas subtypes is essential for the practice of personalized and targeted medicine. The clinical management for these two NSCLC subtypes is different due to their different molecular properties and histological origins. Immunohistochemistry (IHC) markers such is TTF-1 play a key role in the differentiation of lung adenocarcinomas and squamous cell carcinomas. However, immunohistochemistry is a complex process involving many critical steps and the reliability of results depends on the standardization of the assay as well as the appropriate interpretation. Different laboratories use different reagents and different IHC approaches for the detection of TTF-1 in lung cancer tumors. Here we describe an automated IHC protocol used in our laboratory for the detection of TTF-1 in formalin-fixed, paraffin-embedded (FFPE) tissue sections from lung tumors.

Automated Immunohistochemistry Assay for the Detection of Napsin-A in Formalin-Fixed Paraffin-Embedded Tissues from Lung Tumors.

Immunohistochemistry (IHC) enables the selective detection of proteins in cells of formalin-fixed-paraffin-embedded (FFPE) tissue sections. This technique plays a key role in the identification and classification of primary lung cancer tumors through the evaluation of the expression of the aspartic proteinase Napsin-A. However, immunohistochemistry is a complex process involving many critical steps and the lack of standardization as well as inappropriate analytical conditions may contribute to inconsistent results between laboratories. Automated immunohistochemistry addresses this issue by ensuring the quality and the reproducibility of the results among different laboratories. Here we describe an automated IHC protocol used in our laboratory for the detection of Napsin-A in FFPE lung tissue sections.

Intratumoral Injection of Clostridium novyi-NT Spores in Patients with Treatment-refractory Advanced Solid Tumors.

PURPOSE: Intratumorally injected Clostridium novyi-NT (nontoxic; lacking the alpha toxin), an attenuated strain of C. novyi, replicates within hypoxic tumor regions resulting in tumor-confined cell lysis and inflammatory response in animals, which warrants clinical investigation. PATIENTS AND METHODS: This first-in-human study (NCT01924689) enrolled patients with injectable, treatment-refractory solid tumors to receive a single intratumoral injection of C. novyi-NT across 6 dose cohorts (1 × 104 to 3 × 106 spores, 3+3 dose-escalation design) to determine dose-limiting toxicities (DLT), and the maximum tolerated dose. RESULTS: Among 24 patients, a single intratumoral injection of C. novyi-NT led to bacterial spores germination and the resultant lysis of injected tumor masses in 10 patients (42%) across all doses. The cohort 5 dose (1 × 106 spores) was defined as the maximum tolerated dose; DLTs were grade 4 sepsis (n = 2) and grade 4 gas gangrene (n = 1), all occurring in three patients with injected tumors >8 cm. Other treatment-related grade ≥3 toxicities included pathologic fracture (n = 1), limb abscess (n = 1), soft-tissue infection (n = 1), respiratory insufficiency (n = 1), and rash (n = 1), which occurred across four patients. Of 22 evaluable patients, nine (41%) had a decrease in size of the injected tumor and 19 (86%) had stable disease as the best overall response in injected and noninjected lesions combined. C. novyi-NT injection elicited a transient systemic cytokine response and enhanced systemic tumor-specific T-cell responses. CONCLUSIONS: Single intratumoral injection of C. novyi-NT is feasible. Toxicities can be significant but manageable. Signals of antitumor activity and the host immune response support additional studies of C. novyi-NT in humans.

Low viscosity highly concentrated injectable nonaqueous suspensions of lysozyme microparticles.

Subcutaneous injection of concentrated protein and peptide solutions, in the range of 100-400 mg/mL, is often not possible with a 25- to 27-gauge needle, as the viscosity can be well above 50 cP. Apparent viscosities below this limit are reported for suspensions of milled lysozyme microparticles up to nearly 400 mg/mL in benzyl benzoate or benzyl benzoate mixtures with safflower oils through a syringe with a 25- to 27-gauge needle at room temperature. These apparent viscosities were confirmed using a cone-and-plate rheometer. The intrinsic viscosity regressed from the Kreiger-Dougherty model was only slightly above the Einstein value of 2.5, indicating the increase in viscosity relative to that of the solvent was caused primarily by the excluded volume. Thus, the increases in viscosity from electrical double layer interactions (electroviscous effects), solvation of the particles, or deviations of the particle shape from a spherical geometry were minimal, and much smaller than typically observed for proteins dissolved in aqueous solutions. The small electroviscous effects are expected given the negligible zeta potential and thin double layers in the low dielectric constant organic solvent. The suspensions were resuspendable after a year, with essentially constant particle size after two months as measured by static light scattering. The lower apparent viscosities for highly concentrated protein suspensions relative to protein solutions, coupled with these favorable characteristics upon resuspension, may offer novel opportunities for subcutaneous injection of therapeutic proteins.

Difference in sarcopenia prevalence and associated factors according to 2010 and 2018 European consensus (EWGSOP) in elderly patients with type 2 diabetes mellitus.

OBJECTIVES: The aim of this study was to establish the prevalence of sarcopenia and associated factors in elderly patients with type 2 diabetes mellitus (DM) according to 2010 (EWGSOP1) and 2018 (EWGSOP2) European consensus. DESIGN: Cross-sectional study. PARTICIPANTS: Elderly outpatients ≥60 years with type 2 DM and able to walk were recruited at the DM ambulatory care center of a public hospital in Porto Alegre from 2017 to 2018. MATERIALS AND METHODS: The diagnosis of sarcopenia was performed according to EWGSOP1 and EWGSOP2. Muscle mass (MM) was assessed using bioelectrical impedance (BIA). Muscle strength (MS) was assessed using the handgrip strength (HS) test and physical performance (PP) by timed-up-and-go (TUG) test. RESULTS: We included 242 patients with 68.3 ± 5.6 years, 54% women, 78% white, DM duration 14(8-22) years, BMI 29.5 ± 4.5 kg/m2, and HbA1c 7.8 ± 1.5%. Overall prevalence of sarcopenia was 21%. In EWGSOP1 it was 16.9%. The GLM Poisson model was used to assess sarcopenia. Male sex increased the prevalence of sarcopenia by 33% (3.330 [1.747-6.350]; p < .001), and walking >5401 steps/day had a protective effect of 70% for the prevalence of sarcopenia (0.306 [0.127-0.739]; p = .029). Finally, age had an impact of 6% on prevalence of sarcopenia (1.06 [1.015-1.108]; p = .009) according to EWGSOP1. On the other hand, the prevalence was 7%, women had more sarcopenia (88%), and BMI was lower in the sarcopenic group when defined according to EWGSOP2. CONCLUSIONS: The prevalence of sarcopenia was more than double when comparing EWGSOP1 (16.9%) and EWGSOP2 (7%). We believe that the difference in prevalence is due to modifications in MM and MS criteria. According to EWGSOP1, walking may have protective role in the prevalence of sarcopenia in elderly type 2 DM individuals.

Two distinct motifs within the p53 transactivation domain bind to the Taz2 domain of p300 and are differentially affected by phosphorylation.

The tumor suppressor p53 functions as a transcriptional activator for many genes, including several key genes involved in cell cycle arrest and apoptosis. Following DNA damage-induced stress, p53 undergoes extensive posttranslational modification, resulting in increased stability and activity. Two critical cofactors for p53-mediated transactivation are the histone acetyltransferase paralogues CREB-binding protein (CBP) and p300. The N-terminal transactivation domain of p53 interacts with several domains of CBP/p300, including the Taz2 domain. Here, we report the effects of specific p53 phosphorylations on its interaction with the Taz2 domain of p300. Using a competitive fluorescence anisotropy assay, we determined that monophosphorylation of p53 at Ser(15) or Thr(18) increased the affinity of p53(1-39) for Taz2, and diphosphorylations at Ser(15) and Ser(37) or Thr(18) and Ser(20) further increased the affinity. In addition, we identified a second binding site for Taz2 within p53 residues 35-59. This second site bound Taz2 with a similar affinity as the first site, but the binding was unaffected by phosphorylation. Thus, p53 posttranslational modification modulates only one of the two binding sites for p300 Taz2. Further investigation of Taz2 binding to p53(1-39) or p53(35-59) by isothermal titration calorimetry indicated that upon complex formation, the change in heat capacity at constant pressure, DeltaC(p), was negative for both sites, suggesting the importance of hydrophobic interactions. However, the more negative value of DeltaC(p) for Taz2 binding to the first (-330 cal/(mol.K)) compared to the second site (-234 cal/(mol.K)) suggests that the importance of nonpolar and polar interactions differs between the two sites.

The importance of being flexible: the case of basic region leucine zipper transcriptional regulators.

Large volumes of protein sequence and structure data acquired by proteomic studies led to the development of computational bioinformatic techniques that made possible the functional annotation and structural characterization of proteins based on their primary structure. It has become evident from genome-wide analyses that many proteins in eukaryotic cells are either completely disordered or contain long unstructured regions that are crucial for their biological functions. The content of disorder increases with evolution indicating a possibly important role of disorder in the regulation of cellular systems. Transcription factors are no exception and several proteins of this class have recently been characterized as premolten/molten globules. Yet, mammalian cells rely on these proteins to control expression of their 30,000 or so genes. Basic region:leucine zipper (bZIP) DNA-binding proteins constitute a major class of eukaryotic transcriptional regulators. This review discusses how conformational flexibility "built" into the amino acid sequence allows bZIP proteins to interact with a large number of diverse molecular partners and to accomplish their manifold cellular tasks in a strictly regulated and coordinated manner.

Structure of the Taz2 domain of p300: insights into ligand binding.

CBP and its paralog p300 are histone acetyl transferases that regulate gene expression by interacting with multiple transcription factors via specialized domains. The structure of a segment of human p300 protein (residues 1723-1836) corresponding to the extended zinc-binding Taz2 domain has been investigated. The crystal structure was solved by the SAD approach utilizing the anomalous diffraction signal of the bound Zn ions. The structure comprises an atypical helical bundle stabilized by three Zn ions and closely resembles the solution structures determined previously for shorter peptides. Residues 1813-1834 from the current construct form a helical extension of the C-terminal helix and make extensive crystal-contact interactions with the peptide-binding site of Taz2, providing additional insights into the mechanism of the recognition of diverse transactivation domains (TADs) by Taz2. On the basis of these results and molecular modeling, a hypothetical model of the binding of phosphorylated p53 TAD1 to Taz2 has been proposed.