Eradication of persistent reovirus infection from a B-cell hybridoma.

The 2G10 B-cell hybridoma was found to be persistently infected with reovirus serotype 3 (RV3). The persistently infected 2G10 culture produced approximately 1 x 10(8) plaque-forming units of virus per milliliter of culture lysate, and a majority of cells in the culture were infected, as determined by infectious center assay and immunocytochemistry. Cure of the persistent infection was achieved by passaging 2G10 cells for 33 days (12 passages) in medium containing polyclonal anti-RV3 antiserum and a monoclonal antibody specific for the RV3 attachment protein. After several passages in antibody-free medium, cured 2G10 cells had (1) nondetectable levels of RV3 in cell-culture lysates, (2) no infectious centers per 3 x 10(5) cells, (3) no immunocytochemically detectable RV3 antigen, and (4) no detectable reovirus-specific RNA by reverse transcription-polymerase chain reaction amplification. Additionally, mice inoculated with cured 2G10 cell lysates did not generate antibodies directed against RV3. These observations demonstrate that persistent reovirus infection of a B-cell hybridoma can be cured by passage in medium containing anti-reovirus antibodies and suggest that the maintenance of this persistent infection is dependent on horizontal cell-to-cell transmission of virus in the culture.

Acetylcholine receptor synthesis from membrane polysomes.

We have established conditions for the fractionation of cytoplasmic and membrane-bound polyribosomes from the clonal mouse cell line BC3H-1. Polyribosome fractions are obtained in good yield and purity. They are active in protein synthesis when incubated with nuclease-treated rabbit reticulocyte lysates, and we have demonstrated that the cytoplasmic and membrane-bound fractions direct the synthesis of distinctly different sets of proteins. Using immunoprecipitation and sodium dodecyl sulfate gel analysis, we have shown that the membrane-bound but not the cytoplasmic polyribosomes direct the synthesis of two protein species (Mr = 39,000 nd 42,000) which are homologous to the native alpha subunit of acetylcholine receptor. Peptide maps suggest that the two species synthesized in vitro may correspond to the nonglycosylated and glycosylated forms, respectively, of the alpha subunit.

The subunit structure of alpha-acetohydroxyacid isomeroreductase from Salmonella typhimurium.

Alpha-Acetohydroxyacid isomeroreductase from Salmonella typhimurium has a native molecular weight of 220,000. The constituent polypeptide chains exhibit anomalous but unimodal electrophoretic migration on sodium dodecyl sulfate-urea polyacrylamide gels. The subunit molecular weight, determined by sedimentation equilibrium in 6 M guanidine hydrochloride, is 57,000. The apparent tetrameric nature of the native enzyme was confirmed by determining the types of oligomers formed upon cross-linking with dimethylsebacimidate. Analysis of tryptic peptides suggests that the polypeptide chains have an identical amino acid sequence. Carbohydrate analysis, ultraviolet absorption spectrum, and atomic absorption spectrum are consistent with the lack of cobalamine and cobalt. The Michaelis constants are as follows: alpha-acetolactate, 2.9 x 10-4 M; alpha-aceto-alpha-hydroxybutyrate, 7.8 x 10-4 M; NADPH, 1.5 x 10-5 M; Mg2+, 7.7 x 10-4 M. The catalytic constants (molecules of substrate catalyzed per min per molecules of enzyme) for alpha-acetolactate and alpha-aceto-alpha-hydroxybutyrate are 1,100 and 4,700, respectively. Comparative tryptic peptide analysis and immunological analysis show that alpha-acetohydroxyacid isomero-reductase and biosynthetic L-threonine deaminase bear no structural relationship and therefore rule out a "shared structure" hypothesis for the putative involvement of L-threonine deaminase in the synthesis of alpha-acetohydroxyacid isomeroreductase.

Threonine deaminase from Salmonella typhimurium. Relationship between regulatory sites.

Kinetic analysis of the biosynthetic threonine deaminase, EC 4.2.1.16, from Samonella typhimurium yields hyperbolic substrate saturation curves in the absence of, and higher order substrate saturation curves in the presence of, L-isoleucine. L-Valine reverses this effect of L-isoleucine by restoring the hyperbolic substrate saturation curves. The inhibition of enzyme activity and the reversal of valine stimulation is a function of a second order concentration of L-isoleucine, whereas antagonism of inhibition is a function of first order concentration of valine. The antagonistic effects on enzyme activity of L-isoleucine and of L-valine appear as competitive in diagnostic plots. Threonine deaminase possesses two L-isoleucine binding sites (Kd equals 3.6 muM) and one L-valine binding site (Kd equals 26 muM); the binding of these ligands appear competitive. Exclusion of L-valine requires the binding of 2 molecules of L-isoleucine whereas binding of a single L-valine molecule prevents the binding of 2 L-isoleucine molecules. Cooperative binding of L-isoleucine is not observed under any of the conditions tested. Two cases, expressed in terms of modified Adair equations and based upon the assumption that L-threonine also serves as an activator ligand which binds to the L-valine site, are presented. Case I states that liganding of the activator sites must percede substrate-binding at the active site, and Case II states that the activator site liganding is required solely for reactivation of the L-isoleucine-inhibited enzyme. Analysis of kinetic data by a curve-fitting process suggests that Case II described the relationship between the activator site and the L-isoleucine sites. An enzymatically inactive derivative of threonine deaminase, prepared by reduction with borohydride, binds isoleucine and valine in a manner similar to native holoenzyme. Binding of L-threonine and L-valine to the derivatized enzyme is competitive. The Kd for threonine binding is 3 mM, which is in excellent agreement with the Kd determined by the curve fitting process. It is concluded that the modulation of threonine deaminase activity is wrought by interaction between inhibitor sites and an activator site rather than inhibitor and active sites and that induced transitions rather than concerted transitions more adequately describe the underlying regulatory principle.

Biodegradative L-threonine deaminase of Salmonella typhimurium.

The threonine deaminase formed under anaerobic conditions by Salmonella typhimurium is induced by l-serine and l-threonine, is catabolite repressible, requires cyclic adenosine 3',5'-monophosphate for its synthesis and adenylic acid for optimal activity, and is immunologically different from biosynthetic threonine deaminase.