In vitro methylation of bacterial chemotaxis proteins: characterization of protein methyltransferase activity in crude extracts of Salmonella typhimurium

A specific in vitro assay was developed for the protein carboxyl methyltransferase that is involved in the chemotactic behavior of Salmonella typhimurium. This cytosolic enzyme catalyzes an S-adenosyl-L-methionine-dependent methyl esterification of glutamyl residues on a class of 60,000-dalton inner-membrane proteins. The activity was found to display a pH optimum of 6.5 and be sensitive to the concentration of salts in the assay medium. No detectable activity was found towards a variety of other proteins which serve as substrates for mammalian and other bacterial carboxyl methyltransferases. This assay was used to quantitate the methylation of the 60,000-dalton methyl-accepting proteins in response to chemoeffectors. Small but reproducible concentration-dependent changes in the initial rates of in vitro methylation were observed with chemotactic attractants and repellents. The specific methyltransferase activity was found to be absent in several mutants in flagellar synthesis (fla-), suggesting that the synthesis of this enzyme is coordinately regulated with that of flagellin and basal bodies. The hydrodynamic properties of the enzyme in crude extracts were determined by gel filtration and sucrose velocity gradient centrifugation, and a native molecular weight of 41,000 was calculated from these data.

The effect of protein modification reagents on the chemotactic response in Salmonella typhimurium

Several classes of reagents that covalently modify proteins have been shown to inhibit the ability of Salmonella typhimurium to reverse the direction of its flagellar rotation. Such reversal normally allows the bacterium to tumble and reorient its movement in a more favorable direction. These reagents include those that react with amino, guanidino, sulfhydryl, and disulfide groups on proteins. At high concentrations, most of these compounds also cause the paralysis of flagellar rotation. Tumbling in bacterial chemotaxis has been shown to be dependent on the methylation of a class of membrane proteins. The effects of these reagents in an in vitro methylation system have been studied. The results obtained suggest that most of these compounds are probably not acting on intact cells by inhibiting the activities of the cheR methyltransferase or the methyl-accepting proteins.

DE. The effect of protein modification reagents on the chemotactic response in Salmonella typhimurium

Several classes of reagents that covalently modify proteins have been shown to inhibit the ability of Salmonella typhimurium to reverse the direction of its flagellar rotation. Such reversal normally allows the bacterium to tumble and reorient its movement in a more favorable direction. These reagents include those that react with amino, guanidino, sulfhydryl, and disulfide groups on proteins. At high concentrations, most of these compounds also cause the paralysis of flagellar rotation.Tumbling in bacterial chemotaxis has been shown to be dependent on the methylation of a class of membrane proteins. The effects of these reagents in an in vitro methylation system have been studied. The results obtained suggest that most of these compounds are probably not acting on intact cells by inhibiting the activities of the cheR methyltransferase or the methyl-accepting proteins.

Membrane receptors for aspartate and serine in bacterial chemotaxis

High affinity binding sites for serine and aspartate have been characterized in membranes from Salmonella typhimurium and Escherichia coli. Greater than 80% of these sites have been identified as chemotaxis receptors. Mutants lacking binding sites for these amino acids have been shown to have corresponding defects in taxis. The substrate specificity of each of the receptors in Salmonella is very high; most analogs of serine and aspartate do not bind to these receptor sites and do not affect chemotaxis. The transport of these amino acids is apparently not related to chemotaxis. At least 2500 serine receptors and 1200 aspartate receptors with dissociation constants of about 5 microM are present in the membrane fraction of logarithmically growing cells.

Protein asymmetry in the inner membrane of rat liver mitochondria

At least three groups of polypeptides of the inner membrane of rat liver mitochondria have been shown to be exposed to the exterior surface by several techniques: lactoperoxidase-catalyzed iodination of mitochondria and inner membrane/matrix vesicles (mitoplasts), reaction of mitochondria and mitoplasts with the membrane-impermeable diazonium salt of sulfanilic acid, and controlled proteolysis of mitoplasts. These classes of proteins, separated by dodecyl sulfate gel electrophoresis, have polypeptide molecular weights of 73,000, 31,000, and 26,000. In addition, four other groups have been shown to be exposed to the exterior surface by at least one of these techniques: these components have polypeptide molecular weights of 130,000, 87,000, 16,000, and 10,500. A class of proteins, which makes up 50 to 60% of the total mitochondrial protein and which can be easily extracted from mitoplasts by freeze-thaw fractionation or other procedures designed to separate "matrix" protein from "membrane" protein, is shown not to be exposed to the outer surface of the inner membrane by these techniques. This class of proteins contains polypeptides of various molecular weights and includes the major 165,000 molecular weight polypeptide, identified with carbamyl phosphate synthetase.

A major polypeptide component of rat liver mitochondria: carbamyl phosphate synthetase

One of the major components of rat liver mitochondria detected by gel electrophoresis in sodium dodecyl sulfate is a 165,000 molecular weight polypeptide that makes up 15 to 20% of the total mitochondrial protein. This component appears to be a single molecular species. Evidence is presented here for the identification of this protein with the polypeptide chain of a urea cycle enzyme, carbamoylphosphate synthetase I (EC 2.7.2.5). The 165,000 molecular weight polypeptide was solubilized from mitochondria with Triton X-100 and purified to 90% homogeneity by DEAE-cellulose chromatography. This component co-migrated with carbamyl phosphate synthetase activity when mitochondrial proteins were separated by gel filtration or sucrose gradient centifugation. The identification of the 165,000 molecular weight polypeptide with this activity was also supported by the presence or absence of this protein in a variety of rat tissue mitochondria, in liver and kidney mitochondria from various ureotelic and nonureotelic species, and in fetal rat liver mitochondria.

The size and detergent binding of membrane proteins

Sucrose density gradient centrifugation has been used to measure the binding of Triton X-100 above its critical micellar concentration to a variety of purified membrane and non-membrane proteins. In addition, binding studies were done on the three proteins below the critical micellar concentration of detergent to distinguish between the interaction of proteins with detergent monomers and detergent micelles. A procedure is described for the calculation of the molecular weight of these Triton X-100 protein complexes and measurements were made for opsin, plasma low density lipoprotein, the (Na-+ plus K-+)-dependent adenosine triphosphatase, the human red blood cell major sialoglycoprotein (PAS-1) and the human red blood cell minor glycoprotein (bandIII). These proteins behave as monomers or dimers in detergent and bind between 0.28 and 1.12 g of detergent per g of protein. A general method is also present for calculating the molecular size and shape of impure membrane proteins in detergent. Finally, Triton X-100 was shown to replace bound Na dodecyl-SO4 on the minor glycoprotein of the red blood cell.