Expression and epitope tagging of the membrane anchor subunit (DmsC) of Escherichia coli dimethyl sulfoxide reductase.

Escherichia coli dimethyl sulfoxide reductase is a heterotrimer comprising a catalytic subunit (DmsA), an electron transfer subunit (DmsB) and an integral membrane anchor subunit (DmsC). DmsC is not antigenic and the production of antibodies to this subunit has not been successful. We have tagged DmsC at the C-terminus with a dystrophin-specific amino acid sequence (dysp) to which antibodies are readily available. We were able to use this tagging technique to monitor expression and localization of DmsC in E. coli and non-muscle eukaryotic cells. Growth properties of wild-type E. coli, strain HB101, overexpressing DmsC:dysp suggest that the expression of DmsC is lethal to E. coli. The lethality could be overcome by utilizing an E. coli F0F1 ATPase mutant as the host. Growth conditions of culture density, duration of induction, temperature of incubation after induction and media conditions were investigated to optimize DmsC:dysp accumulation levels. In order to alleviate the problem arising from the toxicity of DmsC, expression in eukaryotic tissue culture was also explored. A plasmid expressing DmsC:dysp was transfected into COS-1 or McA-RH777 cells. The presence of expressed DmsC:dysp was confirmed using specific anti-dysp antibodies and immunofluorescence microscopy analysis revealed that the DmsC:dysp was localized to the endoplasmic reticulum. Expression of DmsC:dysp did not appear to be toxic to the eukaryotic cells. These data suggest methodologies to overcome lethality problems associated with the overexpression of integral membrane proteins like DmsC.

Endoplasmic reticulum form of calreticulin modulates glucocorticoid-sensitive gene expression.

Calreticulin is a ubiquitously expressed Ca2+-binding protein of the endoplasmic reticulum (ER), which inhibits DNA binding in vitro and transcriptional activation in vivo by steroid hormone receptors. Transient transfection assays were carried out to investigate the effects of different intracellular targeting of calreticulin on transactivation mediated by glucocorticoid receptor. BSC40 cells were transfected with either calreticulin expression vector (ER form of calreticulin) or calreticulin expression vector encoding calreticulin minus leader peptide, resulting in cytoplasmic localization of the recombinant protein. Transfection of BSC40 cells with calreticulin expression vector encoding the ER form of the protein led to 40-50% inhibition of the dexamethasone-sensitive stimulation of luciferase expression. However, in a similar experiment, but using the calreticulin expression vector encoding cytoplasmic calreticulin, dexamethasone-stimulated activation of the luciferase reporter gene was inhibited by only 10%. We conclude that the ER, but not cytosolic, form of calreticulin is responsible for inhibition of glucocorticoid receptor-mediated gene expression. These effects are specific to calreticulin, since overexpression of the ER lumenal proteins (BiP, ERp72, or calsequestrin) has no effect on glucocorticoid-sensitive gene expression. The N domain of calreticulin binds to the DNA binding domain of the glucocorticoid receptor in vitro; however, we show that the N+P domain of calreticulin, when synthesized without the ER signal sequence, does not inhibit glucocorticoid receptor function in vivo. Furthermore, expression of the N domain of calreticulin and the DNA binding domain of glucocorticoid receptor as fusion proteins with GAL4 in the yeast two-hybrid system revealed that calreticulin does not interact with glucocorticoid receptor under these conditions. We conclude that calreticulin and glucocorticoid receptor may not interact in vivo and that the calreticulin-dependent modulation of the glucocorticoid receptor function may therefore be due to a calreticulin-dependent signaling from the ER.

Phosphorylation of the carboxyl-terminal region of dystrophin.

Dystrophin is a protein product of the gene responsible for Duchenne and Becker muscular dystrophy. The protein is localized to the inner surface of sarcolemma and is associated with a group of membrane (glyco)proteins. Dystrophin links cytoskeletal actins via the dystrophin-associated protein complex to extracellular matrix protein, laminin. This structural organization implicates the role of dystrophin in stabilizing the sarcolemma of muscle fibers. Precisely how dystrophin functions is far from clear. The presence of an array of isoforms of the C-terminal region of dystrophin suggests that dystrophin may have functions other than structural. In agreement, many potential phosphorylation sites are found in the C-terminal region of dystrophin, and the C-terminal region of dystrophin is phosphorylated both in vitro and in vivo by many protein kinases, including MAP kinase, p34cdc2 kinase, CaM kinase, and casein kinase, and is dephosphorylated by calcineurin. The C-terminal domain of dystrophin is also a substrate for hierarchical phosphorylation by casein kinase-2 and GSK-3. These observations, in accordance with the finding that the cysteine-rich region binds to Ca2+, Zn2+, and calmodulin, suggest an active involvement of dystrophin in transducing signals across muscle sarcolemma. Phosphorylation-dephosphorylation of the C-terminal region of dystrophin may play a role in regulating dystrophin-protein interactions and (or) transducing signal from the extracellular matrix via the dystrophin molecule to the cytoskeleton.

Characterization of the recombinant C-terminal domain of dystrophin: phosphorylation by calmodulin-dependent protein kinase II and dephosphorylation by type 2B protein phosphatase.

We report that the C-terminal domain of skeletal muscle dystrophin expressed as a fusion protein with glutathione S-transferase (designated GST-CT-1) is a substrate for Ca2+/calmodulin-dependent phosphorylation and dephosphorylation. GST-CT-1 and GST-CT-1F (GST-CT-1 truncated by 20-25 residues) were phosphorylated by Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). The stoichiometries of phosphorylation by CaM kinase II were 1.65 mol of Pi/mol of GST-CT-1 and 0.39 mol of Pi/mol of GST-CT-1F, respectively, suggesting that the principal site(s) of phosphorylation is (are) located in the C-terminal 20-25 residues that are missing from GST-CT-1F. The GST-CT-1 fusion protein was phosphorylated on both serine and threonine residues, whereas GST-CT-1F was phosphorylated only on serine. CaM kinase II-phosphorylated GST-CT-1 and GST-CT-1F were efficiently dephosphorylated by calcineurin, a Ca2+/calmodulin-dependent protein phosphatase (type 2B protein phosphatase). Importantly, calcineurin was found to be associated with a purified sarcolemmal membrane preparation enriched in dystrophin. Type 2A protein phosphatase isolated from smooth muscle (SMP-I) and its catalytic subunit (SMP-ic) also dephosphorylated GST-CT-1, but were less active toward these substrates than was calcineurin. Type 2C phosphatase (SMP-II) and type 1 protein phosphatases [SMP-III, SMP-IV, and myosin-associated phosphatase (PP1M) of smooth muscle and skeletal muscle protein phosphatase 1c] were ineffective in dephosphorylating the C-terminal region of dystrophin.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of dystrophin. The carboxyl-terminal region of dystrophin is a substrate for in vitro phosphorylation by p34cdc2 protein kinase.

In this paper, we report that p34cdc2 protein kinase phosphorylates recombinant fragments of skeletal muscle dystrophin with a maximal incorporation of 1.8 mol of Pi/mol of protein. Phosphorylation of both serine and threonine residues occurs within the carboxyl-terminal 201 amino acids of dystrophin, with phosphothreonine localized to within 25 residues of the carboxyl terminus. Supporting these in vitro studies, we also show that native dystrophin is phosphorylated by p34cdc2 kinase in isolated sarcolemmal vesicles. Sequence analysis indicates two consensus sites for p34cdc2 protein kinase within the carboxyl-terminal 201 amino acids of dystrophin. Importantly, neither of these sites is conserved in dystrophin-related protein, and only one site is conserved in the 71-kDa alternative product of the Duchenne muscular dystrophy gene, despite an otherwise extremely high degree of sequence conservation between these proteins. Importantly, in this study we also show that dystrophin is phosphorylated in vivo in rat skeletal muscle primary cultures, and we suggest that further investigation of both in vivo and in vitro phosphorylation of this protein will comprise an important part in determination of its function(s).