A simple method for stabilizing protein-sulfhydryl groups during SDS-gel electrophoresis

Synthesis and processing of Semliki Forest virus-specific nonstructural proteins in vivo and in vitro

A large short-lived virus-specific nonstructural protein with an apparent molecular weight of about 250000 (nsp250) has been isolated from cells infected with the temperature-sensitive mutants ts-4 and ts-6 of the Semliki Forest virus. nsp250 contained all peptides characteristic of the two previously identified nonstructural precursor proteins, nsp155 and nsp135, as revealed by limited proteolysis with Staphylococcus aureus V8 protease. Thus nsp250 is probably the translational product of the 5' two-thirds of the 42-S RNA genome which codes for the virus-specific nonstructural proteins. A second viral nonstructural precursor protein, nsp220, was also characterized by peptide mapping. This protein contained all the peptides of nsp155, and several but not all of the peptides of nsp135. Some peptides were demonstrated which possibly are derived from ns60, the only nonstructural protein not yet isolated. Small amounts of proteins with identical mobility to nsp250 and nsp220 were synthesized at 38 degrees C in micrococcal-nuclease-treated rabbit reticulocyte lysate in response to virion 42-S RNA from the ts-6 mutant. The product of the wild-type 42-S RNA in vitro contained, in addition to nsp220 and nsp155, polypeptides which comigrated with ns86, ns72 and ns70, indicating processing of the translational product. The authenticity of nsp220, nsp155 and ns70 synthesized in vitro was confirmed by limited proteolysis with V8 protease.

Differential postendocytotic compartmentation in Xenopus oocytes is mediated by a specifically bound ligand

Xenopus laevis oocytes were used as a model system to study the intracellular fate of proteins incorporated by endocytosis. We found that the intracellular stability and compartmentation of proteins incorporated by receptor-mediated endocytosis differed substantially from that of proteins incorporated by nonspecific endocytosis. After its uptake, the specifically sequestered yolk precursor protein vitellogenin was converted to the yolk proteins lipovitellin and phosvitin which were stable with time (up to 13 days in culture). In contrast, nonspecifically incorporated albumin (125I-labeled or 3H-labeled bovine serum albumin) was rapidly degraded. To determine whether the differential stability of these proteins was related to their entry into different postendocytotic compartments, we examined the intracellular transfer pathways taken by these proteins. The transfer of vitellogenin from coated vesicles to the yolk platelets was found to involve a secondary compartment formed by the fusion of the incoming endosomes. This compartment, termed transitional yolk bodies (TYB), underwent a progressive condensation until it attained its terminal density (1.21 g/cm3) after approximately 1.5 hr. The fusion of the TYB with the yolk platelets then occurred coincidentally with the time at which vitellogenin was proteolytically processed into the yolk proteins within the TYB. When the proteolytic cleavage of vitellogenin was blocked there was no fusion of the two compartments. In contrast, we found that albumin incorporated in the absence of vitellogenin was directly transferred from endosomes to yolk platelets without the formation of, or fusion with a secondary compartment. However, when oocytes were exposed simultaneously to albumin and vitellogenin both proteins followed identical routes of compartmentation (that of vitellogenin) with no evidence of direct transfer of either protein to the yolk platelets. These results suggest that the incorporation of a specifically bound ligand can result in the formation of a unique intracellular compartment. Moreover, since yolk platelets were able to fuse only with vesicles lacking occupied receptors (in the case of albumin alone) or with a compartment in which the specific ligand had been proteolytically cleaved and presumably released from its receptor (in the case of VTG), we suggest that occupied receptors can act as a transmembrane signal which directs the postendocytotic compartmentation of proteins.

Partial purification and characterization of a macromolecule which enhances fluoride activation of adenylate cyclase

Fluoride activation of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] is significantly enhanced (2 to 5 times) by a protein factor isolated from rat brain. The fluoride-dependent adenylate cyclase stimulator (FCS) is nondialyzable, trypsin-labile, and stable at 90 degrees C for 10 min. FCS stimulates adenylate cyclase activity only in the presence of NaF (2-25 mM) and this effect is independent of added GTP, 5'-guanylylimidodiphosphate, or calcium. FCS has been purified roughly 3000-fold from a 12,000 X g supernatant fraction of rat brain homogenate. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis and sucrose density gradient sedimentation suggest that FCS is a monomer with an apparent Mr of 59,000. Isoelectric focusing indicates FCS has a pI of 8.9. FCS from rat brain stimulates fluoride-activated adenylate cyclase from a variety of cell types, and FCS can also be isolated from rat liver. The effects of FCS are not reversed by washing membranes when the membranes and FCS are preincubated with NaF. The Km of adenylate cyclase for ATP and the fluoride concentration causing half-maximal activation are unchanged by FCS; however, FCS increases the Vmax by 2.5-fold. FCS may act to increase the catalytic efficiency of fluoride-activated complexes of the GTP-binding unit with adenylate cyclase or to enhance the formation of additional active complexes.

Isolation and characterization of two molecular forms of octopine dehydrogenase from Pecten maximus L

Two forms, A and B, of octopine dehydrogenase from Pecten muximus L. straited adductor muscles were separated by ion-exchange chromatography and purified to homogeneity. Their kinetic properties were similar and, among others, the mnemonical behaviour, previously found for octopine dehydrogenase, was confirmed. Structural features, determined by polyacrylamide gel electrophoresis with and without sodium dodecylsulfate, amino acid composition, immunological tests and heat stability were compared. The only differences between the two forms are their charge and their sensibility to various chemical treatments. Assays of reciprocal conversion of the two forms by oxidation, reduction or deamidation failed. No tissue specificity and no relation to any physiological conditions could be observed. However, a constant ratio A:B = 1:4 was statistically found in crude extracts as well as in the purified enzyme. It therefore seems possible to assume that the two forms of octopine dehydrogenase pre-exist in living P. maximus, although their genetic origin has to be established.

The anomalous migration during two-dimensional gel electrophoresis of eukaryotic ribosomal proteins with oxidised thiol groups

Two-dimensional gel electrophoresis of eukaryotic ribosomal proteins under conditions in which thiol groups are not maintained in a reduced state has been found to cause marked changes in the migration of certain proteins, apparently due to the formation of intramolecular disulphide bridges. The results suggest that certain previously designated eukaryotic ribosomal proteins are not, in fact, unique species.

Partial purification of the sugar carrier of intestinal brush border membranes. Enrichment of the phlorizin-binding component by selective extractions

The [3H] phlorizin-binding component of brush border vesicles was enriched in situ by negative purification. Several procedures, known to effect selective solubilization of membrane components, were used separately or in combination to remove proteins unrelated to the binding. Deoxycholate ruptured the vesicles and released 67% of their protein, thereby increasing the specific [3H] phlorizin-binding activity of the pellet three-to fourfold. Extracting the deoxycholate-pellets with either NaI or alkaline solutions released up to 38% of the deoxycholate-insoluble protein without significantly affecting phlorizin binding. The polypeptide composition of the membranes at the different stages was analyzed by NaDodSO4-polyacrylamide gel electrophoresis. A number of polypeptides present in the original vesicles could be ruled out as essential components of the [3H] phlorizin binding entity. Intact and deoxycholate-treated vesicles were subjected to proteolytic attack. Papain liberated sucrase and isomaltase from intact vesicles, but affected neither other Coomassie-stained bands nor phlorizin binding. Neither the protein composition nor the binding properties of sealed vesicles were influenced by trypsin or chymotrypsin. However, all the proteolytic enzymes tested on deoxycholate-treated membranes substantially reduced [3H] phlorizin binding and produced concomitantly the disappearance of several bands from the electrophoretic profile. Pretreatment of vesicles with papain, followed by deoxycholate extraction and incubation in alkaline media, increased the specific binding activity of the membranes up to ninefold by removing close to 90% of the protein. A limited number of polypeptides are suggested as possible candidates for the glycoside-binding site of intestinal brush borders.

Titration curves of polypeptide chains by combined isoelectric focusing-electrophoresis in 8 M urea

Titration curves of reduced and alkylated polypeptide chains can be successfully performed in 8 M urea-polyacrylamide gel plates by electrophoresis perpendicular to a stationary stack of focused carrier ampholytes. All buffers and thiol reagents with pK values in the range pH 3--10 should be removed, since their pH-dependent ionization affects the migration and apparent pI values of the protein chains. No blurring of the patterns below pH 4.5 is observed, as usually found in titration curves in the absence of urea, thus allowing the direct titration of Glu and Asp residues. It is not possible by the present technique to titrate any group below pH ca. 3 and above pH ca. 10, due to the lack of suitable carrier ampholytes and to a "flooding" phenomenon, with concomitant identical electrophoretic mobility for all protein species, irrespective of their relative pI values and amino acid composition. The "electrophoretic titration curves" thus obtained were well correlated with the overall amino acid composition of the polypeptide chains analyzed.

HORDEIN VARIATION IN WILD (HORDEUM SPONTANEUM) AND CULTIVATED (H. VULGARE) BARLEY

The storage protein hordein contains two major groups of polypeptides which are highly polymorphic in barley, and in its evolutionary progenitor Hordeum spontaneum Koch. Crosses between the two species showed that the complex electrophoretic phenotypes within the two groups of polypeptides are governed by codominant alleles at two corresponding loci, Hor-1 and Hor-2, which are moderately linked (11% ± 2). In natural populations of the wild species, the two complex hordein loci were much more polymorphic than the allozyme loci. Furthermore, the variation at these two loci was highly correlated so that individuals differ from one another at both loci much more frequently than expected from the allele frequencies at the individual loci. Considerable hordein variation was also present in Composite Cross XXI, and there was evidence of reassortment of patterns by the seventeenth generation. Thus the complex hordein loci, with their extreme diversity and linkage disequilibrium, are ideal markers for monitoring evolutionary processes in both natural, or composite cross populations.