Phosphorylation of the myosin heavy chain. Its effect on actin-activated Mg2+-stimulated ATPase in leukaemic myeloblasts.

Myosin purified from a murine myeloid leukaemia cell line (M1) that had been incubated with [32P]orthophosphate incorporated 32P into the heavy, but not the light, chain. When the heavy chain was dephosphorylated by bacterial alkaline phosphatase, myosin that had low actin-activated ATPase activity gained higher activity only in the presence of the light-chain kinase. In the absence of the light-chain kinase, however, the Mg2+-stimulated ATPase activity of myosin was not activated by actin, regardless of phosphatase treatment. These results indicate that the activity of M1 myosin ATPase is regulated by phosphorylation of both the light and heavy chains. A scheme for this regulation by phosphorylation is presented and discussed.     

Cytosol promotes the guanine nucleotide-induced release of the alpha subunit of Gi2 from the membrane of mouse mastocytoma P-815 cells

Translocation of the alpha subunit of Gi2 from the membrane to the cytosol was studied in mouse mastocytoma P-815 cells. To monitor Gi2 alpha the membrane (300,000 x g pellet) was [32P]ADP-ribosylated with pertussis toxin. Incubation of the [32P]ADP-ribosylated membrane with guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) caused a small release (10%) of [32P]ADP-ribosylated Gi2 alpha from the membrane. Whereas cytosol (300,000 x g supernatant) alone had no ability to release the [32P]ADP-ribosylated Gi2 alpha from the membrane, it markedly augmented the release induced by GTP gamma S, about 50% of the total [32P]ADP-ribosylated Gi2 alpha being released by 30 min. The GTP gamma S-induced release and its enhancement by the cytosol were specific for GTP and GTP gamma S. When the cytosol was boiled this promoting activity was lost. The [32P]ADP-ribosylated Gi2 alpha released by the cytosol plus GTP gamma S from the membrane was eluted as a single peak corresponding to a molecular weight of about 100,000 from an Ultrogel AcA 44 column. In contrast, the [32P]ADP-ribosylated Gi2 alpha released by GTP gamma S alone was eluted at the position of Mr = 40,000, but it was eluted at the position of Mr = about 100,000 when it was incubated with the cytosol. Furthermore, Gi2 alpha purified from bovine lung also behaved in a similar way on gel filtration. The addition of thrombin, a stimulant of histamine secretion from mast cells, to mastocytoma cells drastically induced the translocation of Gi2 alpha from the membrane to the cytosol in a pertussis toxin-sensitive manner. These results taken together demonstrate that the cytosol contains some factor(s) that promotes the release of GTP-activated Gi2 alpha from the membrane and that the released Gi2 alpha exists in the cytosol as a soluble complex with unidentified component(s) in mastocytoma cells.     

Immunoelectron microscopic study of a new D-amino acid oxidase-immunoreactive subcompartment in rat liver peroxisomes.

We report the presence of a new subcompartment in rat liver peroxisomal matrix in which only D-amino acid oxidase is localized and other matrix enzymes are absent. By electron microscopic observation, the rat liver peroxisome has generally been considered to consist of a single limiting membrane, an electron-dense crystalline core, and a homogeneous matrix. Immunohistochemical staining for D-amino acid oxidase by the protein A-gold technique revealed the presence of a small area in the matrix that was immunoreactive for the enzyme and was less electron-dense than the surrounding matrix. The localization of D-amino acid oxidase in this small area of the peroxisomal matrix was confirmed by immunoelectron microscopy on freeze-substituted tissues processed without chemical fixation. To analyze the characteristics of the electron-lucent area, immunoreactivity for various peroxisomal enzymes, including catalase, acyl-CoA oxidase, enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase bifunctional protein, 3-ketoacyl-CoA thiolase, L-alpha-hydroxy acid oxidase (isozyme B), and glycolate oxidase (isozyme A), was assayed. The electron-lucent area was negative for all of these. By double staining for D-amino acid oxidase and catalase, using colloidal gold particles of different sizes, these enzymes were shown to be located in separate areas in the matrix.     

Ligand-binding characteristics of rat serum-type mannose-binding protein (MBP-A). Homology of binding site architecture with mammalian and chicken hepatic lectins

Sugar-binding characteristics of rat serum mannose-binding protein (MBP) were studied using the carbohydrate-recognition domain of this protein expressed from a cloned cDNA. To assess the binding affinity of various test compounds, they were added as inhibitors in a binding assay in which 125I-MBP was incubated with yeast cells and the extent of binding was estimated from the radioactivity associated with the pelleted cells. The results of such inhibition assays suggest that MBP has a small binding site which is probably of the trough-type. The 3- and 4-OH of the target sugar are indispensable, while the 6-OH is not required. These characteristics are shared by the rat hepatic lectin and chicken hepatic lectin, both of which are C-type lectins containing carbohydrate-recognition domains highly homologous to that of MBP. Apparently, the related primary structures of these lectins give rise to similar gross architecture of their binding sites, despite the fact that each exhibits different sugar binding specificities.     

Interaction of organic chemicals with P-glycoprotein in the adrenal gland, kidney, and a multidrug-resistant KB cell.

P-glycoprotein (P-gp) is thought to mediate the transport of anti-cancer drugs and to be responsible for the multidrug-resistant (MDR) phenotype in tumor cells. However, the function of P-gp in normal tissues is still not well understood. We present evidence indicating that the active efflux of several structurally unrelated organic compounds is mediated by P-gp in multidrug-resistant KB (KB-C2) cells and that these compounds interact with P-gp in the kidney and adrenal gland. The photoactive radioactive calcium channel blocker [3H]azidopine labels a protein of approximately 140 kDa in crude membrane fractions from human kidney and adrenal gland and a 130-kDa protein from bovine adrenal gland. These photolabeled proteins are immunoprecipitated with an anti-P-gp antibody. Photolabeling is inhibited by vinblastine, reserpine, and several other organic chemicals. These data indicate that the kidney and adrenal gland express P-gp (or a protein closely related to P-gp) that can interact with several organic compounds and that the P-gp expressed in these tissues has a drug-binding site similar to that of P-gp in KB-C2 cells. Our findings thus strongly support the hypothesis that P-gp can transport a wide variety of organic chemicals as well as anti-cancer drugs and that one of the physiological functions of P-gp is the excretion of certain classes of organic compounds.     

Na+(Li+)/Branched-chain amino acid cotransport inPseudomonas aeruginosa

Summary A transport system for branched-chain amino acids (designated as LIV-II system) inPseudomonas aeruginosa requires Na⁺ for its operation. Coupling cation for this system was identified by measuring cation movement during substrate entry using cation-selective electrodes. Uptakes of Na⁺ and Li^– were induced by the imposition of an inwardly-directed concentration gradient of leucine, isoleucine, or valine. No uptake of H^– was found, however, under the same conditions. In addition, effects of Na⁺ and Li⁺ on the kinetic property of the system were examined. At chloride salt concentration of 2.5mm, values of apparentK _m andV _max for leucine uptake were larger in the presence of Na⁺ than Li⁺. These results indicate that the LIV-II transport system is a Na⁺(Li⁺)/substrate cotransport system, although effects of Na⁺ and Li⁺ on kinetics of the system are different.     

Purification and characterization of NADPH-dependent cytosolic 3,5,3'-triiodo-L-thyronine binding protein in rat kidney

The NADPH-dependent cytosolic 3,5,3'-triiodo-L-thyronine(T3)-binding protein (CTBP) has been purified over 30,000-fold from rat kidney by using charcoal extraction, Mono Q-Sepharose, Blue Sepharose CL-6B, and Sephacryl S-200 column chromatography. Purified CTBP had a sedimentation coefficient of 4.7 S, Stokes radius of 32.5A, and calculated molecular weight of 58,000. The apparently homogeneous protein consisted of a single polypeptide chain with Mr of 58,000 as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Scatchard analysis of T3 binding showed that NADPH increases maximal binding capacity without changes in the affinity constant (Ka = 2.43 X 10(9) M-1). Double reciprocal analysis of NADPH and binding capacity gave maximal binding capacity of 16,400 pmol/mg of CTBP, Mr = 58,000. The order of affinity of iodothyronine analogues to purified CTBP was as follows: L-T3 = D-T3 greater than triiodothyroacetic acid greater than L-thyroxine. [125I]T3 bound to purified CTBP spontaneously dissociated from CTBP at 20 degrees C (t 1/2 = 22 min) in the absence of NADPH, whereas the dissociation was not observed in the presence of NADPH. The optimal pH for T3 binding was 7.2-7.5 Na+, K+, Ca2+, and Mg2+ (0-200 mM) did not influence T3 binding to CTBP. The purified CTBP did not bind to DNA and was not adsorbed to concanavalin A-Sepharose.     

Isolation and characterization of deletion mutants affected in early genes of bacteriophage ?80

Summary When Escherichia coli cells that had been irradiated with ultraviolet light were infected with bacteriophage 80, five major (pE, pB, pA, pC and pD) and two minor (pU and pV) proteins were found to be synthesized during early stages of infection. The genss coding for the five major proteins were mapped on the 80 chromosome using various deletion mutants which lacked the capacity to synthesize some or all the major proteins. The size and positions of all the deletions were determined by gel electrophoresis of EcoRI digests of phage DNA and by electron microscopy of heteroduplexes between DNAs of the deletion and wild-type phage. The five major proteins designated pE(25K), pB(40K), pA(45K), pC(34K) and pD(31K) were shown to be encoded in this order presumably by a single operon that was located at 60.2–67.4% on the 80 genome. These proteins were found to be involved in phage recombination. The absence of pE or pB resulted in a Red^– phenotype and the absence of three proteins (pE, pB and pA) resulted in a Fec^– phenotype. The exact positions of the genes for the minor proteins pU(29K) and pV(26K) have not been determined.