Synthesis of colicin Ia neoglycoproteins: tools towards glyco-engineering of bacterial cell surfaces.

Colicins are antimicrobial proteins produced by certain strains of Escherichia coli that function as offensive weapons against closely-related competitor strains. Their bactericidal properties and narrow bacterial targeting range has made them of therapeutic interest. Furthermore, the applications of engineered non-bactericidal colicins are of interest as a cell surface-directed protein anchor for decorating E. coli with biomolecules. We previously demonstrated that an engineered non-bacteriocidal colicin E9 could be used to label bacterial cells with multiple biomolecules including glycans. Herein we extend our approach to colicin Ia, constructing mannose-presenting colicin la neoglycoproteins, through N-terminal organocatalyst-mediated protein aldol ligation (OPAL), or maleimide ligation targeting an internal cysteine. This work further highlights the potential utility of engineered colicins for non-genetic glyco-engineering of the E. coli cell surface.

Structural basis of DNA packaging by a ring-type ATPase from an archetypal viral system.

Many essential cellular processes rely on substrate rotation or translocation by a multi-subunit, ring-type NTPase. A large number of double-stranded DNA viruses, including tailed bacteriophages and herpes viruses, use a homomeric ring ATPase to processively translocate viral genomic DNA into procapsids during assembly. Our current understanding of viral DNA packaging comes from three archetypal bacteriophage systems: cos, pac and phi29. Detailed mechanistic understanding exists for pac and phi29, but not for cos. Here, we reconstituted in vitro a cos packaging system based on bacteriophage HK97 and provided a detailed biochemical and structural description. We used a photobleaching-based, single-molecule assay to determine the stoichiometry of the DNA-translocating ATPase large terminase. Crystal structures of the large terminase and DNA-recruiting small terminase, a first for a biochemically defined cos system, reveal mechanistic similarities between cos and pac systems. At the same time, mutational and biochemical analyses indicate a new regulatory mechanism for ATPase multimerization and coordination in the HK97 system. This work therefore establishes a framework for studying the evolutionary relationships between ATP-dependent DNA translocation machineries in double-stranded DNA viruses.

Subcellular dynamics and functional activity of the cleaved intracellular domain of the Na+ channel ß1 subunit.

The voltage-gated Na+ channel ß1 subunit, encoded by SCN1B, regulates cell surface expression and gating of α subunits and participates in cell adhesion. ß1 is cleaved by α/ß and γ-secretases, releasing an extracellular domain and intracellular domain (ICD), respectively. Abnormal SCN1B expression/function is linked to pathologies including epilepsy, cardiac arrhythmia, and cancer. In this study, we sought to determine the effect of secretase cleavage on ß1 function in breast cancer cells. Using a series of GFP-tagged ß1 constructs, we show that ß1-GFP is mainly retained intracellularly, particularly in the endoplasmic reticulum and endolysosomal pathway, and accumulates in the nucleus. Reduction in endosomal ß1-GFP levels occurred following γ-secretase inhibition, implicating endosomes and/or the preceding plasma membrane as important sites for secretase processing. Using live-cell imaging, we also report ß1ICD-GFP accumulation in the nucleus. Furthermore, ß1-GFP and ß1ICD-GFP both increased Na+ current, whereas ß1STOP-GFP, which lacks the ICD, did not, thus highlighting that the ß1-ICD is necessary and sufficient to increase Na+ current measured at the plasma membrane. Importantly, although the endogenous Na+ current expressed in MDA-MB-231 cells is tetrodotoxin (TTX)-resistant (carried by Nav1.5), the Na+ current increased by ß1-GFP or ß1ICD-GFP was TTX-sensitive. Finally, we found ß1-GFP increased mRNA levels of the TTX-sensitive α subunits SCN1A/Nav1.1 and SCN9A/Nav1.7. Taken together, this work suggests that the ß1-ICD is a critical regulator of α subunit function in cancer cells. Our data further highlight that γ-secretase may play a key role in regulating ß1 function in breast cancer.

A Tale of Two Bioconjugations: pH Controlled Divergent Reactivity of Protein α-oxo-Aldehydes in Competing α-oxo-Mannich and Catalyst-Free Aldol Ligations.

Site-selective chemical methods for protein bioconjugation have revolutionized the fields of cell and chemical biology through the development of novel protein/enzyme probes bearing fluorescent, spectroscopic, or even toxic cargos. Herein, we report two new methods for the bioconjugation of α-oxo aldehyde handles within proteins using small molecule aniline and/or phenol probes. The "α-oxo-Mannich" and "catalyst-free aldol" ligations both compete for the electrophilic α-oxo aldehyde, which displays pH divergent reactivity proceeding through the "Mannich" pathway at acidic pH to afford bifunctionalized bioconjugates, and the "catalyst-free aldol" pathway at neutral pH to afford monofunctionalized bioconjugates. We explore the substrate scope and utility of both of these bioconjugations in the construction of neoglycoproteins, in the process formulating a mechanistic rationale for how both pathways intersect with each other at different reaction pH's.

Design and synthesis of fluorescent 7-deazaadenosine nucleosides containing π-extended diarylacetylene motifs.

C-modified 7-deazaadenosines containing a diphenylacetylene moiety have been synthesised using cross-coupling approaches. The C-modified nucleosides exhibit remarkable fluorescence properties, including high quantum yields. Solvatochromic studies show a near linear correlation between the Stokes shift and solvent polarity which is indicative of intramolecular charge transfer. DFT calculations have allowed us to correlate the experimentally observed photophysical properties with the calculated HOMO-LUMO energy gaps within a series of real and model compounds.

Pd(0)/Cu(I)-mediated direct arylation of 2'-deoxyadenosines: mechanistic role of Cu(I) and reactivity comparisons with related purine nucleosides.

Pd/Cu-mediated direct arylation of 2'-deoxyadenosine with various aryl iodides provides 8-arylated 2'-deoxyadenosine derivatives in good yields. Following significant reaction optimization, it has been determined that a substoichiometric quantity of piperidine (secondary amine) in combination with cesium carbonate is necessary for effective direct arylation. The general synthetic protocol allows lower temperature direct arylations, which minimizes deglycosylation. The origin of the piperidine effect primarily derives from the in situ generation of Pd(OAc)(2)[(CH(2))(5)NH](2). Various copper(I) salts have been evaluated; only CuI provides good yields of the 8-arylated-2'-deoxyadenosines. Copper(I) appears to have a high binding affinity for 2'-deoxyadenosine, which explains the mandatory requirement for stoichiometric amounts of this key component. The conditions are compared with more general direct arylation protocols, e.g., catalytic Pd, ligand, acid additives, which do not employ copper(I). In each case, no detectable arylation of 2'-deoxyadenosine was noted. The conformational preferences of the 8-aryl-2'-deoxyadenosine products have been determined by detailed spectroscopic (NMR) and single crystal X-ray diffraction studies. Almost exclusively, the preferred solution-state conformation was determined to be syn-C2'-endo (ca. 80%). The presence of a 2-pyridyl group at the 8-position further biases the solution-state equilibrium toward this conformer (ca. 88%), due to an additional H-bond between H1' and the pyridyl nitrogen atom. The Pd/Cu catalyst system has been found to be unique for adenosine type substrates, the reactivity of which has been placed into context with the reported direct arylations of related 1H-imidazoles. The reactivity of other purine nucleosides has been assessed, which has revealed that both 2'-deoxyguanosine and guanosine are incompatible with the Pd/Cu-direct arylation conditions. Both substrates appear to hinder catalysis, akin to the established inhibitory effects in Suzuki cross-couplings with arylboronic acids.

Metabolism of the viable mammalian embryo: quietness revisited.

This review examines the 'Quiet Embryo Hypothesis' which proposes that viable preimplantation embryos operate at metabolite or nutrient turnover rates distributed within lower ranges than those of their less viable counterparts. The 'quieter' metabolism consistent with this hypothesis is considered in terms of (i) 'functional' quietness; the contrasting levels of intrinsic metabolic activity in different cell types as a consequence of their specialized functions, (ii) inter-individual embryo/cell differences in metabolism and (iii) loss of quietness in response to environmental stress. Data are reviewed which indicate that gametes and early embryos function in vivo at a lower temperature than core body temperature, which could encourage the expression of a quiet metabolism. We call for research to determine the optimum temperature for mammalian gamete/embryo culture. The review concludes by examining the key role of reactive oxygen species, which can induce molecular damage, trigger a cellular stress response and lead to a loss of quietness.