Telegraph noise in coupled quantum dot circuits induced by a quantum point contact.

Charge detection utilizing a highly biased quantum point contact has become the most effective probe for studying few electron quantum dot circuits. Measurements on double and triple quantum dot circuits is performed to clarify a back action role of charge sensing on the confined electrons. The quantum point contact triggers inelastic transitions, which occur quite generally. Under specific device and measurement conditions these transitions manifest themselves as bounded regimes of telegraph noise within a stability diagram. A nonequilibrium transition from artificial atomic to molecular behavior is identified. Consequences for quantum information applications are discussed.

Pokeweed mitogen-stimulated human lymphocytes fused to LICR-2 (HMY2) generate human-human hybridomas producing monoclonal IgG antibodies reactive to human breast carcinoma and malignant melanoma.

Human-human hybridomas were generated using pokeweed mitogen-stimulated lymphocytes from the regional lymph nodes of cancer patients by fusion to the LICR-2 human myeloma cell line. A total of 35 fusions, using the regional lymph node lymphocytes of cancer patients, resulted in hybrid growth in 23% of wells plated with 21 IgG ELISA positive clones, 6 of which have maintained stable human monoclonal antibody production. Mononuclear cells were separated on Ficoll-Paque and grown for 3-4 days in 1% pokeweed mitogen and fused to the LICR-2 human myeloma cell line. Human-human hybridoma producing membrane reactive IgG antibodies have been isolated and react to the following cancers: breast; melanoma. Twenty-seven fusions from 8 breast carcinoma patients resulted in 13 ELISA positive IgGs, 3 of which were stable after cloning. A total of 5,071 wells were plated after polyethylene glycol fusion with resultant hybrid growth in 1210 wells (24% hybrid growth) after hypoxanthine-aminopterin-thymidine selection. In 8 fusions using regional lymph node lymphocytes of other types of cancer, including 6 fusions using lymphocytes from malignant melanoma patients, there were 1,580 wells plated with positive growth in 20% of the wells (311 wells). Of these, 8 clones were ELISA positive and 3 stable clones all producing IgG anti-melanoma antibody were isolated. The overall hybrid frequency was 43 x 10(-7) fused lymphocytes (39 x 10(-7) non-breast and 45 x 10(-7) breast). A total of 21 IgG-producing clones were identified to crude membranes of allogeneic tumor cell lines and stable antibody production was achieved for 6 (29% stable clones).(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and partial purification of the insulin-responsive glucose transporter from 3T3-L1 adipocytes.

The glucose transporter in 3T3-L1 adipocytes has been identified as a polypeptide of average Mr 51000 by means of its reaction with antibodies raised against the purified human erythrocyte glucose transporter and by photolabeling with [3H]cytochalasin B. The finding that the antibodies immunoprecipitated the photolabeled polypeptide demonstrated that both methods detected the same polypeptide. The 3T3-L1 adipocyte glucose transporter has been partially purified. The main steps in the purification procedure were the preparation of salt-washed cellular membranes, Triton X-100 solubilization, and immunoaffinity chromatography on affinity-purified antibodies against the human erythrocyte transporter. A simple method of affinity purification of these antibodies, which consists of adsorption from serum onto protein-depleted erythrocyte membranes and release with acid, and an assay for the 3T3-L1 adipocyte transporter polypeptide, which employs immunoblotting, have been developed.

Identification of the glucose transporter in rat skeletal muscle.

The glucose transporter in the plasma membrane of rat skeletal muscle has been identified by two approaches. In one, the transporter was detected as the polypeptide that was differentially labeled by photolysis with [3H]cytochalasin B in the presence of L- and D-glucose. [3H]Cytochalasin B is a high-affinity ligand for the transporter that is displaced by D-glucose. In the other, the transporter was detected by means of its reaction with rabbit antibodies against the purified glucose transporter from human erythrocytes. By both procedures, the transporter was found to be a polypeptide with a mobility corresponding to a molecular weight of 45,000-50,000 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Relative stability of membrane proteins in Escherichia coli.

The relative stability of membrane proteins in Escherichia coli was investigated to determine whether these proteins are degraded at heterogeneous rates and, if so, whether the degradative rates are correlated with the sizes or charges of the proteins. Cells growing in a glucose-limited chemostat with a generation time of 15 h were labeled with [(14)C]leucine. After allowing 24 h for turnover of (14)C-labeled proteins, the cells were labeled for 15 min with [(3)H]leucine. By this protocol, the rapidly degraded proteins have a high ratio of (3)H to (14)C, whereas the stable proteins have a lower ratio. The total cell envelope fraction was collected by differential centrifugation, and the proteins were separated by two-dimensional polyacrylamide gel electrophoresis. The relative ratio for each protein was determined by dividing its (3)H/(14)C ratio by the (3)H/(14)C ratio of the total membrane fraction. Although most of the 125 membrane proteins had relative ratios close to the average for the total membrane fraction, 19 varied significantly from this value. These differences were also observed when the order of addition of [(14)C]leucine and [(3)H]leucine was reversed. In control cultures labeled simultaneously with both isotopes, the relative ratios of these 19 proteins were similar to that of the total membrane fraction. Thirteen of these proteins had low relative ratios, which suggested that they were more stable than the average protein. An experiment in which the normal labeling procedure was followed by a 60-min chase period in the presence of excess unlabeled leucine suggested that the low relative ratios of 3 of these 13 proteins may be due to a slow post-translational modification step. Six membrane proteins had high relative ratios, which indicated that they were degraded rapidly. In contrast to the relationships found for soluble proteins in mammalian cells, there were no strong correlations between the degradative rates and either the isoelectric points or the molecular weights of membrane proteins in E. coli.

Relative stability of intracellular proteins in bacterial cells.

The relative stabilities of soluble and membrane proteins were examined in growing Escherichia coli cells. In contrast to mammalian cells, we found no correlations between the isoelectric points or molecular weights of E. coli proteins and their degradative rates. The soluble proteins with short half-lives tended to be degraded preferentially in vitro by trypsin or chymotrypsin. The stability of membrane proteins in vivo was correlated with in vitro sensitivity to chymotrypsin but not to trypsin. In the total membrane fraction, endogenous proteolytic activity varied with growth conditions. This activity was inhibited by o-phenanthroline, EDTA and dithiothreitol suggesting that one or more metallo-proteinases were present. Membrane proteinase activity was also inhibited by phenethyl alcohol, a membrane perturbant. The abundance of the membrane proteins that were most labile in vivo was dependent on growth conditions. The most labile protein accumulated in the outer membrane with an inverse relationship to growth rate.