Binding proteins and membrane transport

Model system to study classical nuclear export signals

Signal-mediated protein transport through the nuclear pore complex is of considerable interest in the field of molecular pharmaceutics. Nuclear localization signals can be used to target genes/antisense delivery systems to the nucleus. Studying nuclear export is useful in enhancing the expression and the efficiency of action of these therapeutic agents. The mechanism of nuclear import has been well studied and most of the proteins participating in this mechanism have been identified. The subject of nuclear export is still in the initial stages, and there is a considerable amount of uncertainty in this area. Two main export receptors identified so far are Exportin 1 (Crm1) and Calreticulin. Crm1 recognizes certain leucine-rich amino acid sequences in the proteins it exports called classical nuclear export signals. This paper describes a model system to study, identify, and establish these classical nuclear export signals using green fluorescent protein (GFP). Two putative export signals in the human progesterone receptor (PR) and the strongest nuclear export signal known (from mitogen activated protein kinase kinase [MAPKK]) were studied using this model system.

Influence of divalent cations in protein crystallization

We have tested the effect of several cations in attempts to crystallize the ligand-bound forms of the leucine/isoleucine/valine-binding protein (LIVBP) (M(r) = 36,700) and leucine-specific binding protein (LBP) (M(r) = 37,000), which act as initial periplasmic receptors for the high-affinity osmotic-shock-sensitive active transport system in bacterial cells. Success was achieved with Cd2+ promoting the most dramatic improvement in crystal size, morphology, and diffraction quality. This comes about 15 years after the ligand-free proteins were crystallized. Nine other different divalent cations were tried as additives in the crystallization of LIVBP with polyethylene glycol 8000 as precipitant, and each showed different effects on the crystal quality and morphology. Cd2+ produced large hexagonal prism crystals of LIVBP, whereas a majority of the cations resulted in less desirable needle-shaped crystals. Zn2+ gave crystals that are long rods with hexagonal cross sections, a shape intermediate between the hexagonal prism and needle forms. The concentration of Cd2+ is critical. The best crystals of the LIVBP were obtained in the presence of 1 mM CdCl2, whereas those of LBP, with trigonal prism morphology, were obtained at a much higher concentration of 100 mM. Both crystals diffract to at least 1.7 A resolution using a conventional X-ray source.

Effects of heat and amino acid supplementation on the uptake of arginine and its incorporation into proteins in Escherichia coli

Hyperthermic treatment reduces protein synthesis and modifies amino acid transport in Escherichia coli. The present study examined the role of nutrient availability on these processes. Cultures of E. coli in log phase were aliquoted into growth medium with or without complete amino acid supplementation and exposed to 37, 44, or 48 degrees C for 10 min. Amino acid supplementation increased radiolabelled arginine uptake at 48 degrees C when compared with unsupplemented cells. Exposure to 48 degrees C also reduced protein synthesis in both groups by at least 50% as reflected by labelled arginine incorporation. Two-dimensional gel electrophoresis indicated that this heat-related decrement in synthesis was most apparent in basic proteins. Total density analysis of the fluorographs demonstrated reductions in basic proteins of 15% at 44 degrees C and 89% at 48 degrees C, while acidic proteins only showed an 80% reduction at 48 degrees C. Amino acid supplementation appears to raise the baseline, but not to modify the final results of hyperthermia-induced inhibition of protein synthesis. The sensitivity of basic protein synthesis seems to be a key event in this process.

Effects of heat and other agents on amino acid uptake in Escherichia coli

In Escherichia coli K1060 grown at 37 degrees C we observed that the uptake of both L-[3H]leucine and L-[35S]methionine was inhibited by exposure of the cells to 48 degrees C. The calcium channel blockers diltiazem and verapamil, and the anti-arrhythmic agent quinidine, inhibited the uptake of L-[3H]leucine at both 37 degrees C and 48 degrees C. Verapamil also inhibited the uptake of L-[35S]methionine at 37 degrees C, but at 48 degrees C protected against some of the heat-induced decrease in the uptake of this amino acid. The local anaesthetic procaine markedly inhibited the uptake of both labelled amino acids at temperatures between 37 degrees C and 48 degrees C. Amino acid uptake and cell killing were not correlated.

Crystallization and preliminary X-ray crystallographic data of a histidine-binding protein from Escherichia coli

Histidine-binding protein, purified from periplasmic space of Escherichia coli K12, has been crystallized in a form suitable for X-ray analysis. Crystals of average size 0.3 mm x 0.15 mm x 0.15 mm have been grown by the hanging-drop method, with ammonium sulfate as precipitant. The space group if I4(1)22, with the unit cell dimensions a = b = 119.1 A; c = 151.8 A; Vm = 2.7 A3/dalton. There appear to be two protein subunits of molecular weight 25,000 each in the asymmetric unit.

Effects of tunicamycin and monensin on the distribution of highly phosphorylated proteins in cells infected with herpes simplex virus type 1

New aspects of the distribution of highly phosphorylated proteins in cells infected with herpes simplex virus type 1 (HSV-1) were investigated at the ultrastructural level by the use of drugs which inhibit the glycosylation of viral proteins. The highly phosphorylated proteins were localized by the bismuth tartrate procedure applied on sections of glutaraldehyde-fixed cells embedded in Lowicryl. The drugs employed were tunicamycin, which alters the glycosylation activity of the rough endoplasmic reticulum (RER), and monensin, which blocks the migration of vesicles of the Golgi apparatus (GA) thereby impairing the glycosylation function of the GA. Tunicamycin induced proliferation of RER and the accumulation of highly phosphorylated proteins on its membranes and also impaired GA vesicle maturation and inhibited the usual accumulation of phosphorylated proteins within them. Monensin induced proliferation of the nuclear envelope, including both outer and inner membranes, with bismuth bound to staggered segments of the latter, and also affected the GA in that bismuth-binding proteins were accumulated on the external surface of the swollen vesicles instead of the lumen. These data suggest that an injury of one membrane system, RER or GA, engenders consequential effects on the other. This also supports evidence for an interrelationship between post-translational glycosylation and phosphorylation of proteins in HSV infection.

Cloning, expression, and nucleotide sequence of livR, the repressor for high-affinity branched-chain amino acid transport in Escherichia coli

The livR gene encoding the repressor for high-affinity branched-chain amino acid transport in Escherichia coli has been cloned from a library prepared from the episome F106. The inserted DNA fragment from the initial cloned plasmid, pANT1, complemented two independent, spontaneously derived, regulatory mutations. Subcloning as well as the creation of deletions with Bal31 exonuclease revealed that the entire regulatory region is contained within a 1.1-kb RsaI-SalI fragment. Expression of the pANT plasmids in E. coli minicells showed that the regulatory region encodes one detectable protein with an apparent molecular weight of 21,000. DNA sequencing revealed one open reading frame of 501 bp encoding a protein with a calculated MW of 19,155. The potential secondary structure of the regulatory protein has been predicted and it suggests that the carboxy terminus may fold into three consecutive alpha helices. These results suggest that the livR gene encodes a repressor which plays a role in the regulation of expression of the livJ and the livK transport genes.

Membrane proteins associated with amino acid transport by yeast (Saccharomyces cerevisiae)

Cells of the wild-type yeast (Saccharomyces cerevisiae) strain Y185, grown under conditions that de-repress the formation of a general amino acid permease ('Gap') system, bind delta-N-chloroacetyl[1-(14)C]ornithine; L- and D-amino acid substrates of the general amino acid permease system protect against this binding. The protein responsible is released from the cells by homogenization or by preparation of protoplasts; it is not released by osmotic shock. This protein is virtually absent from the wild-type strain when it is grown under conditions that repress the general amino acid permease system, and is also absent from a Gap- mutant Y185-His3, selected by its resistance to D-amino acids. This mutant and repressed wild-type cells also fail to form a number of membrane proteins elaborated by de-repressed wild-type cells. It is possible that all these proteins are components of the general amino acid permease system.

The relA locus specifies a positive effector in branched-chain amino acid transport regulation

The regulation of branched-chain amino acid transport and periplasmic binding proteins was studied in Escherichia coli strains which were isogenic except for the relA locus, the gene for the "stringent factor," which is responsible for guanosine tetraphosphate synthesis. The strain containing the relA mutation could not be derepressed for the synthesis of leucine transport or binding proteins when shifted from a medium containing all 20 amino acids in excess to one in which leucine was limiting. The relA+ strain showed normal derepression under these conditions.

Branched-chain amino acid transport regulation in mutants blocked in tRNA maturation and transcriptional termination

The regulation of branched-chain amino acid transport and binding protein biosynthesis was studied in Escherichia coli strains containing hisT (the structural gene for pseudouridine synthetase) and rho (the structural gene for the mRNA transcriptional termination factor rho) mutations. The results indicate that the hisT strain cannot be fully derepressed for transport and that the hisT rho double mutant is partially derepressed under excess leucine conditions, but cannot be further derepressed by leucine deprivation. These data are consistent with a model in which fully mature tRNALeu is required for derepression and in which rho interacts with tRNALeu in regulating transport by terminating transcription, especially in excess-leucine growth conditions.

Regulation of branched-chain amino acid transport in Escherichia coli

The repression and derepression of leucine, isoleucine, and valine transport in Escherichia coli K-12 was examined by using strains auxotrophic for leucine, isoleucine, valine, and methionine. In experiments designed to limit each of these amino acids separately, we demonstrate that leucine limitation alone derepressed the leucine-binding protein, the high-affinity branched-chain amino acid transport system (LIV-I), and the membrane-bound, low-affinity system (LIV-II). This regulation did not seem to involve inactivation of transport components, but represented an increase in the differential rate of synthesis of transport components relative to total cellular proteins. The apparent regulation of transport by isoleucine, valine, and methionine reported elsewhere was shown to require an intact leucine, biosynthetic operon and to result from changes in the level of leucine biosynthetic enzymes. A functional leucyl-transfer ribonucleic acid synthetase was also required for repression of transport. Transport regulation was shown to be essentially independent of ilvA or its gene product, threonine deaminase. The central role of leucine or its derivatives in cellular metabolism in general is discussed.