Protein interactions involved in nuclear import of the Agrobacterium VirE2 protein in vivo and in vitro

Agrobacterium, the only known organism capable of trans-kingdom DNA transfer, genetically transforms plants by transferring a segment of its DNA, T-DNA, into the nucleus of the host cell where it integrates into the plant genome. One of the central events in this genetic transformation process is nuclear import of the T-DNA molecule, which to a large degree is mediated by the bacterial virulence protein VirE2. VirE2 is distinguished by its nuclear targeting, which occurs only in plant but not in animal cells and is facilitated by the cellular VIP1 protein. The molecular mechanism of the VIP1 function is still unclear. Here, we used in vitro assays for nuclear import and quantification of protein-protein interactions to directly demonstrate formation of ternary complexes between VirE2, VIP1, and a component of the cellular nuclear import machinery, karyopherin alpha. Our results indicate that VIP1 functions as a molecular bridge between VirE2 and karyopherin alpha, allowing VirE2 to utilize the host cell nuclear import machinery even without being directly recognized by its components.     

Chondroitin sulfate chains on syndecan-1 and syndecan-4 from normal murine mammary gland epithelial cells are structurally and functionally distinct and cooperate with heparan sulfate chains to bind growth factors. A novel function to control binding of midkine, pleiotrophin, and basic fibroblast growth factor

A comparative analysis was carried out of heparan sulfate (HS) and chondroitin sulfate (CS) chains of the ectodomains of hybrid type transmembrane proteoglycans, syndecan-1 and -4, synthesized simultaneously by normal murine mammary gland epithelial cells. Although the HS chains were structurally indistinguishable, intriguingly the CS chains were structurally and functionally distinct, probably reflecting the differential regulation of sulfotransferases involved in the synthesis of HS and CS. The CS chains of the two syndecans comprised nonsulfated, 4-O-, 6-O-, and 4,6-O-disulfated N-acetylgalactosamine-containing disaccharide units and were significantly different, with a higher degree of sulfation for syndecan-4. Functional analysis using a BIAcore system showed that basic fibroblast growth factor (bFGF) specifically bound only to the HS chains of both syndecans, whereas midkine (MK) and pleiotrophin (PTN) bound not only to the HS but also to the CS chains. Stronger binding of MK and PTN to the CS chains of syndecan-4 than those of syndecan-1 was revealed, supporting the structural and functional differences. Intriguingly, removal of the CS chains decreased the association and dissociation rate constants of MK, PTN, and bFGF for both syndecans, suggesting the simultaneous binding of these growth factors to both types of chains, producing a ternary complex that transfers the growth factors to the corresponding cell surface receptors more efficiently compared with the HS chains alone. The involvement of the core protein was also shown in the binding of MK and PTN to syndecan-1, suggesting the possibility of cooperation with the HS and/or CS chains in the binding of these growth factors and their delivery to the cell surface receptors.     

Syndecan-1 and -4 synthesized simultaneously by mouse mammary gland epithelial cells bear heparan sulfate chains that are apparently structurally indistinguishable

Many of the biological functions attributed to cell surface heparan sulfate (HS) proteoglycans, including the Syndecan family, are elicited through the interaction of their HS chains with soluble extracellular molecules. Tightly controlled, cell-specific sulfation and epimerization of HS precursors endows these chains with highly sulfated, iduronate-rich regions, which are major determinants of cytokine and matrix-protein binding and which are interspersed by N-acetylated, poorly sulfated regions. Until this study, there have been no comprehensive structural comparisons made on HS chains decorating simultaneously expressed, but different, syndecan core proteins. In this paper we demonstrate that the HS chains on affinity-purified syndecan-1 and -4 from murine mammary gland cells are essentially identical by a number of parameters. Size determination, disaccharide analyses, enzymatic and chemical scission methods, and affinity co-electrophoresis all failed to reveal any significant differences in fine structure, domain organization, or ligand-binding properties of these HS species. These findings lead us to suggest that the imposition of the fine structure onto HS occurs independently of the core protein to which it is attached and that these core proteins, in addition to the HS chains, may play a pivotal role in the various biological functions ascribed to these macromolecules.     

Inhibition of nuclear import by backbone cyclic peptidomimetics derived from the HIV-1 MA NLS sequence

In the present work we have constructed a series of backbone cyclic peptides, which differed in the amino acid residues located at the C-terminal position of the previously described BCvir peptide (A. Friedler, N. Zakai, O. Karni, Y.C. Broder, L. Baraz, M. Kotler, A. Loyter, C. Gilon, Biochemistry 37 (1998)). BCvir is a cyclic peptide, derived from the nuclear localization signal (NLS) of the human immunodeficiency virus type 1 matrix protein. The majority of the cyclic peptides described here inhibited nuclear import in vitro. The most potent inhibitors were those bearing bulky hydrophobic amino acids such as Leu, Phe or Nal (naphthyl Ala) at the C-terminus. On the other hand, peptides bearing polar amino acid residues such as Asn, Cys or a reduced amide bond were not inhibitory. The present studies demonstrate the importance of a bulky hydrophobic C-terminal side chain and an exocyclic amide bond preceding it, to the inhibitory activity of the NLS-derived BC peptides. Being only inhibitory, these BC peptides resemble classic receptor antagonists.     

Distinct primary translation products from human liver mRNA give rise to secreted and cell-associated forms of complement protein C2

The second component of complement (C2), is a class III major histocompatibility complex gene product and a glycoprotein in the classical complement activating system. Synthesis in the human hepatoma-derived cell line HepG2 results in three intracellular forms: an 84-kDa form secreted in 1-2 h; 79-kDa and 70-kDa forms that remain cell-associated for intervals up to 12 h. All three forms are C2 polypeptides as demonstrated by inhibition of immunoprecipitation with unlabeled C2 and the presence of common major peptide fragments following chymotryptic digestion. The cell-associated forms of C2 are not products of proteolysis as demonstrated by experiments with multiple proteinase inhibitors and by observations of the kinetics of synthesis. Inhibition of core glycosylation by tunicamycin and deglycosylation by acid hydrolysis indicate that the three intracellular C2 polypeptides are glycosylated to a similar extent. Although the 84-kDa form of C2 is susceptible to C1s cleavage, the two cell-associated forms are not. Cell-free biosynthesis by mRNA from HepG2 or human liver results in three primary translation products corresponding to the three unglycosylated forms of C2. These results indicate that HepG2 cells synthesize C2 protein in both secreted and cell-associated forms and that each form is derived from a separate primary translation product.     

Aging and the complexity of cardiovascular dynamics.

Biomedical signals often vary in a complex and irregular manner. Analysis of variability in such signals generally does not address directly their complexity, and so may miss potentially useful information. We analyze the complexity of heart rate and beat-to-beat blood pressure using two methods motivated by nonlinear dynamics (chaos theory). A comparison of a group of healthy elderly subjects with healthy young adults indicates that the complexity of cardiovascular dynamics is reduced with aging. This suggests that complexity of variability may be a useful physiological marker.     

Specific binding of the first enzyme for histidine biosynthesis to the DNA of histidine operon.

Studies were done to examine direct binding of the first enzyme of the histidine biosynthetic pathway (phosphoribosyltransferase) to 32P-labeled phi80dhis DNA and competition of this binding by unlabeled homologous DNA and by various preparations of unlabeled heterologous DNA, including that from a defective phi80 bacteriophage carrying the histidine operon with a deletion of part of its operator region. Our findings show that phosphoribosyltransferase binds specifically to site in or near the regulatory region of the histidine operon. The stability of the complex formed by interaction of the enzyme with the DNA was markedly decreased by the substrates of the enzyme and was slightly increased by the allosteric inhibitor, histidine. These findings are consistent with previous data that indicate that phosphoribosyltransferase plays a role in regulating expression of the histidine operon.     

trans-Recessive mutation in the first structural gene of the histidine operon that results in constitutive expression of the operon.

The first enzyme for histidine biosynthesis, encoded in the hisG gene, is involved in regulation of expression of the histidine operon in Salmonella typhimurium. The studies reported here concern the question of how expression of the histidine operon is affected by a mutation in the hisG gene that alters the allosteric site of the first enzyme for histidine biosynthesis, rendering the enzyme completely resistant to inhibition by histidine. The intracellular concentrations of the enzymes encoded in the histidine operon in a strain carrying such a mutation on an episome and missing the chromosomal hisG gene are three- to fourfold higher than in a strain carrying a wild-type hisG gene on the episome. The histidine operon on such a strain fails to derepress in response to histidine limitation and fails to repress in response to excess histidine. Furthermore, utilizing other merodiploid strains, we demonstrate that the wild-type hisG gene is trans dominant to the mutant allele with respect to this regulatory phenomenon. Examination of the regulation of the histidine operon in strains carrying the feedback-resistant mutation in an episome and hisT and hisW mutations in the chromosome showed that the hisG regulatory mutation is epistatic to the hisT and hisW mutations. These data provide additional evidence that the first enzyme for histidine biosynthesis is involved in autogenous regulation of expression of the histidine operon.     

The ligament of Marshall as a parasympathetic conduit

The objective of the study was to investigate the morphology, distribution, and electrophysiological profile of the autonomic fibers that innervate the ligament of Marshall (LOM). Gross anatomical dissections were performed in 10 dogs. Sections of the left vagus nerve, left stellate ganglion, and the LOM were immunostained to identify adrenergic and cholinergic nerves. Hearts were also stained for acetylcholinesterase to identify epicardial cholinergic nerves. In vivo electrophysiological studies were performed in another 10 dogs before and after LOM ablation. The anatomical examination revealed that the LOM is innervated by a branch of the left vagus. Immunohistochemistry confirmed that these nerve bundles are predominantly cholinergic (cholinergic-to-adrenergic ratio of 12.6 +/- 3.9:1). Cholinergic nerves originating in the LOM were found to innervate surrounding left atrial structures, including the pulmonary veins, left atrial appendage, coronary sinus, and posterior left atrial fat pad. Ablation of the LOM significantly attenuated effective refractory period shortening at distant sites, such as pulmonary veins and left atrial appendage, in response to vagal stimulation (vagal-induced ERP decrease in the left atrium: baseline vs. postablation = 17 vs. 4%; P = 0.0056). In conclusion, the LOM contains a predominance of cholinergic nerve fibers. Cholinergic fibers arising from the LOM innervate surrounding structures and contribute to the electrophysiological profile of the left atrium. These findings may provide a basis for the role of the LOM in the genesis and maintenance of atrial fibrillation.