Characterization of acetohydroxy acid synthase activity in the archaeon Haloferax volcanii

Whereas the biochemistry of acetohydroxy acid synthase has been extensively studied in bacteria and eukaryotes, relatively little is known about the enzyme in archaea, the third kingdom of life. The present study biochemically characterizes acetohydroxy acid synthase activity in the halophilic archaea Haloferax volcanii. In addressing ion requirements, enzyme inhibition and antibody labeling, the results reveal that, except for its elevated salt requirements, the haloarchaeal enzyme is remarkably similar to its bacterial counterpart.

Frequency doubling of ultrashort laser pulses in biological tissues

Theoretical and experimental studies of second-harmonic generation (SHG) in biological tissues was performed by use of ultrashort laser pulses (<1 ps). A simplified one-dimensional model for the generation and the propagation of frequency-doubled light inside tissue was developed. This model was tested in vitro against measurements of pig and chicken tissue and human tooth. The experimental results indicate that the intensity of SHG varies significantly among tissue types and between test sites in individual tissue. Possibilities of using this nonlinear tissue property in imaging and diagnostics are discussed.

The SecA subunit of Escherichia coli preprotein translocase is exposed to the periplasm

SecA undergoes conformational changes during translocation, inserting domains into and across the membrane or enhancing the protease resistance of these domains. We now show that some SecA bound at SecYEG is accessible from the periplasm to a membrane-impermeant probe in cells with a permeabilized outer membrane but an intact plasma membrane.

Endogenous SecA catalyzes preprotein translocation at SecYEG

SecA is found in the cytosol and bound to the plasma membrane of Escherichia coli. Binding occurs either with high affinity at SecYEG or with low affinity to lipid. Domains of 65 and 30 kDa of SecYEG-bound SecA insert into the membrane upon interaction with preprotein and ATP. Azide blocks preprotein translocation, in vivo and in vitro, through interacting with SecA and preventing SecA deinsertion. This provides a measure of the translocation relevance of each form of SecA membrane association. We now report that azide acts exclusively on SecA that is cycling at SecYEG and has no effect on SecA lipid associations. SecA molecules recovered with sucrose gradient-purified inner membrane vesicles ("endogenous" SecA) support translocation at the same rate as "added" SecA molecules bound at SecYEG. Both endogenous and added SecA yield the same proteolytic fragments, which are distinct from those obtained from SecA once it has inserted into membranes at SecYEG or from SecA at lipidic sites. Endogenous and added SecA differ, however, in their resistance to urea extraction. The translocation supported by either endogenous or added SecA is blocked by azide or by antibody to SecY. We conclude that SecA functions in preprotein translocation only through cycling at SecYEG.

125I-labeled fasciculin 2: a new tool for quantitation of acetylcholinesterase densities at synaptic sites by EM-autoradiography

Radio-iodinated fasciculin 2 (Fas2), a polypeptide anticholinesterase toxin from Mamba venom, was used as a new probe for localizing and quantifying acetylcholinesterase (AChE) at mouse neuromuscular junctions (NMJs) by quantitative electron microscope autoradiography. We demonstrate that 125I-Fas2 binds very specifically to the NMJs of mouse sternomastoid muscles, with very little binding to other regions in the muscles. Junctional AChE-site densities obtained from the autoradiograms were similar to those previously obtained for the same muscles using 3H-DFP. The use of 125I-Fas2 with EM-autoradiography is simpler and provides higher resolution and sensitivity, as well as considerably lower non-specific binding than previously attainable with 3H-DFP. The advantages and limitations of this procedure are discussed.

Both an N-terminal 65-kDa domain and a C-terminal 30-kDa domain of SecA cycle into the membrane at SecYEG during translocation

SecA, a 102-kDa hydrophilic protein, couples the energy of ATP binding to the translocation of preprotein across the bacterial inner membrane. SecA function and topology were studied with metabolically labeled [35S]SecA and with inner membrane vesicles from cells that overexpressed SecYEGDFyajC, the integral domain of preprotein translocase. During translocation in the presence of ATP and preprotein, a 65-kDa N-terminal domain of SecA is protected from proteolytic digestion through insertion into the membrane, as previously reported for a 30-kDa C-terminal domain [Economou, A. & Wickner, W. (1994) Cell 78, 835-843]. Insertion of both domains occurs at saturable SecYEGDFyajC sites and is rapidly followed by deinsertion. SecA also associates nonsaturably and unproductively with lipid. In the presence of ATP, yet without involvement of preprotein or SecYEG, lipid-bound SecA forms domains that are protease-resistant and that remain so even upon subsequent membrane disruption. Unlike the [35S]SecA that inserts into the membrane at SecYEGDFyajC as it promotes preprotein translocation, lipid-associated [35S]SecA does not chase from its protease-resistant state upon the addition of excess SecA. The finding that two domains of SecA (which together represent most regions of the polypeptide chain) cycle into the membrane during preprotein translocation, as well as the distinction between the membrane association of SecA at translocation sites of SecYEGDFyajC and at nonproductive lipid sites, are fundamental to the study of the role of SecA in preprotein movement.

The protease-protected 30 kDa domain of SecA is largely inaccessible to the membrane lipid phase

SecA binds to the inner membrane of Escherichia coli through low affinity lipid interactions or with high affinity at SecYEG, the integral domain of preprotein translocase. Upon addition of preprotein and nucleotide, a 30 kDa domain of SecYEG-bound SecA is protected from proteolysis via membrane insertion. Such protection could result from some combination of insertion into the lipid phase, into a proteinaceous environment or across the membrane. To assess the exposure of SecYEG-bound SecA to membrane lipids, a radiolabeled, photoactivatable and lipid-partitioning crosslinker, 3-trifluoromethyl-3-(m[125I]iodophenyl) diazirine benzoic acid ester, was incorporated into inner membrane vesicles. The 30 kDa domain of SecYEG-bound SecA, inserted into the membrane in response to translocation ligands, is 18-fold less labeled than SecY, which is labeled effectively. In contrast, incorporation of the purified 30 kDa SecA fragment into crosslinker-containing detergent micelles or addition of detergent to crosslinker-containing membranes bearing the protease-protected SecA domain readily allows for labeling of this domain. We propose that the protease-inaccessible 30 kDa SecA domain is shielded from the fatty acyl membrane phase by membrane-spanning SecYEG helices and/or is largely exposed to the periplasm.

Libraries from libraries: generation and comparison of screening profiles

A positional scanning tetrapeptide library was chemically modified through alkylation and/or reduction of the amide bonds, thus generating three new combinatorial libraries with physico-chemical properties very different from the parent peptide library ('libraries from libraries'). Specific results were obtained with each of these libraries upon screening in kappa-opioid receptor binding and microdilution antimicrobial assays, illustrating the potential of the 'libraries from libraries' concept for the efficient generation of a variety of chemically diverse combinatorial libraries.

Novel alpha-glucosidase inhibitors identified using multiple cyclic peptide combinatorial libraries

Twenty-six cyclic synthetic peptide combinatorial libraries (disulfides and lactams) of varying size and composition, representing 6.8 x 10(3) to 4.7 x 10(7) individual peptides, were synthesized along with their respective linear analogs. One of the hexapeptide lactam libraries (cyclo[xXxXxN]) was found to have significant alpha-glucosidase inhibitory activity. This library was carried through an iterative process of synthesis and screening, during which all of the five mixture positions (x and X) were successively defined. As the result of this process, potent and selective alpha-glucosidase inhibitors were identified.

Inclusion volume solid-phase synthesis

Solid-phase synthesis of peptides was carried out using only the volume of the solvent included in the swollen solid-phase resin heads (inclusion volume synthesis). This approach enables (i) the use of higher concentrations of activated amino acids, resulting in increased coupling rates, (ii) drastically decreased consumption of solvents, and (iii) the construction of multiple peptide synthesizers having virtually no reaction vessels.

SecYEG and SecA are the stoichiometric components of preprotein translocase

The transport of large preproteins across the Escherichia coli plasma membrane is catalyzed by preprotein translocase, comprised of the peripherally bound SecA subunit and an integrally bound heterotrimeric domain consisting of the SecY, SecE, and SecG subunits. We have now placed the secY, secE, and secG genes under the control of an arabinose-inducible promoter on a multicopy plasmid. Upon induction, all three of the proteins are strongly overexpressed and recovered in the plasma membrane fraction. These membranes show a strong enhancement of 1) translocation ATPase activity, 2) preprotein translocation, 3) capacity for SecA binding, and 4) formation of the membrane-inserted form of SecA. These data establish that SecY, SecE, and SecG constitute the integral membrane domain of preprotein translocase.

Generation and utilization of synthetic combinatorial libraries

The use of combinatorial chemistry is fundamentally changing the pace and scope of basic research and drug discovery. Since the introduction of synthetic peptide libraries several years ago, combinatorial chemistry has proven to be a powerful tool for the generation of immense molecular diversities of peptides, peptidomimetics and new organic compounds. This article briefly reviews methods for the generation and application of combinatorial libraries, with particular emphasis on soluble synthetic combinatorial libraries. The utility of these molecular diversities for basic research and drug discovery has been demonstrated through the identification of numerous highly active compounds such as antigenic peptides, receptor ligands, antimicrobial compounds and enzyme inhibitors.