Further characterization of a novel lectin derived from silkworm faeces; specific binding to immunoglobulins,and activation of immunocytes

In previous studies a novel lectin-like glycoprotein was isolated from silkworm faeces and shown to recognize sugar chains, especially mannose on the cell surface as an epitope, and to cause aggregation of various types of cells in suspension. However, this substance caused detachment and aggregation of only some types of plated cells, such as QM-RSV cells, which are quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus (ts-RSV) at 35.5 °C, a permissive temperature for RSV. As described here, during studies on the mechanism of cell detachment and aggregation of QM-RSV cells by this new lectin, some novel biological activities of the lectin were recognized. This lectin was found to bind to immunoglobulins (Ig) specifically at specific amino acid sequences, not via recognition of their molecular conformation. It also recognized the neural cell adhesion molecule (NCAM), which is one of the members of the Ig-superfamily that have Ig-like domains. Furthermore, it had a strong mitogenic effect on lymphocytes, and also caused about 3-fold of phagocytosis by macrophages within 24 hr after its addition.     

Mutagenic and nonmutagenic bypass of DNA lesions by Drosophila DNA polymerases dpoleta and dpoliota

cDNA sequences were identified and isolated that encode Drosophila homologues of human Rad30A and Rad30B called drad30A and drad30B. Here we show that the C-terminal-truncated forms of the drad30A and drad30B gene products, designated dpoletaDeltaC and dpoliotaDeltaC, respectively, exhibit DNA polymerase activity. dpoletaDeltaC and dpoliotaDeltaC efficiently bypass a cis-syn-cyclobutane thymine-thymine (TT) dimer in a mostly error-free manner. dpoletaDeltaC shows limited ability to bypass a 6-4-photoproduct ((6-4)PP) at thymine-thymine (TT-(6-4)PP) or at thymine-cytosine (TC-(6-4)PP) in an error-prone manner. dpoliotaDeltaC scarcely bypasses these lesions. Thus, the fidelity of translesion synthesis depends on the identity of the lesion and on the polymerase. The human XPV gene product, hpoleta, bypasses cis-syn-cyclobutane thymine-thymine dimer efficiently in a mostly error-free manner but does not bypass TT-(6-4)PP, whereas Escherichia coli DNA polymerase V (UmuD'(2)C complex) bypasses both lesions, especially TT-(6-4)PP, in an error-prone manner (Tang, M., Pham, P., Shen, X., Taylor, J. S., O'Donnell, M., Woodgate, R., and Goodman, M. F. (2000) Nature 404, 1014-1018). Both dpoletaDeltaC and DNA polymerase V preferentially incorporate GA opposite TT-(6-4)PP. The chemical structure of the lesions and the similarity in the nucleotides incorporated suggest that structural information in the altered bases contribute to nucleotide selection during incorporation opposite these lesions by these polymerases.     

Functional domains of the ClpA and ClpX molecular chaperones identified by limited proteolysis and deletion analysis

Escherichia coli ClpA and ClpX are ATP-dependent protein unfoldases that each interact with the protease, ClpP, to promote specific protein degradation. We have used limited proteolysis and deletion analysis to probe the conformations of ClpA and ClpX and their interactions with ClpP and substrates. ATP gamma S binding stabilized ClpA and ClpX such that that cleavage by lysylendopeptidase C occurred at only two sites. Both proteins were cleaved within in a loop preceding an alpha-helix-rich C-terminal domain. Although the loop varies in size and composition in Clp ATPases, cleavage occurred within and around a conserved triad, IG(F/L). Binding of ClpP blocked this cleavage, and prior cleavage at this site rendered both ClpA and ClpX defective in binding and activating ClpP, suggesting that this site is involved in interactions with ClpP. ClpA was also cut at a site near the junction of the two ATPase domains, whereas the second cleavage site in ClpX lay between its N-terminal and ATPase domains. ClpP did not block cleavage at these other sites. The N-terminal domain of ClpX dissociated upon cleavage, and the remaining ClpXDeltaN remained as a hexamer, associated with ClpP, and expressed ATPase, chaperone, and proteolytic activity. A truncated mutant of ClpA lacking its N-terminal 153 amino acids also formed a hexamer, associated with ClpP, and expressed these activities. We propose that the N-terminal domains of ClpX and ClpA lie on the outside ring surface of the holoenzyme complexes where they contribute to substrate binding or perform a gating function affecting substrate access to other binding sites and that a loop on the opposite face of the ATPase rings stabilizes interactions with ClpP and is involved in promoting ClpP proteolytic activity.     

Production of a biologically active epidermal growth factor fusion protein with high collagen affinity

Collagen is generally incapable of capturing polypeptides such as growth factors in a specific manner. In this study, we established a collagen-binding growth factor (FNCBD-EGF) consisting of epidermal growth factor (EGF) and the fibronectin collagen-binding domain. A typical yield of FNCBD-EGF was approximately 200 microg/ml culture in an Escherichia coli expression system. This fusion protein bound to gelatin and fibrillar collagen sponges, and the bound protein was not effectively eluted even with 2 M NaCl. In addition, FNCBD-EGF bound to type I, II, III, or IV collagen-coated plates, and the specificity of binding was confirmed by competitive inhibition using fibronectin. FNCBD-EGF substantially stimulated cell growth after binding to collagen-coated culture plates, whereas EGF had no effect, indicating that this fusion protein acted as a collagen-associated growth factor. In an animal model of impaired wound healing, FNCBD-EGF, but not EGF, was retained with collagen sponges at wound sites 4 d after implantation, and repair of epidermis was observed underneath the sponges. These results suggested that our fusion protein with high collagen affinity would be useful for wound healing.     

Reclassification of Pichia scaptomyzae and Pichia galeiformis

Based on the base composition of nuclear DNA and DNA/DNA hybridization, Pichia galeiformis IFO 10718T was reclassified as a synonym of Pichia manshurica, and Pichia scaptomyzae IFO 1073 1T was confirmed to be a synonym of Pichia membranifaciens. Comparison of 18S rRNA gene sequences indicated that IFO 10731T (P. scaptomyzae) is identical to P. membranifaciens IFO 10215T and IFO 10725, and IFO 10718T (P. galeiformis) is identical to P. manshurica IFO 10726T. These data were consistent with the view that P scaptomyzae and P membranifaciens should be conspecific, as should P. galeiformis and P manshurica. Variation among 26S rRNA gene domain D1/D2 sequences from three P membranifaciens strains indicated that this species encompasses a genetically heterogeneous population.     

Characterization of the stromal protease(s) degrading the cross-linked products of the D1 protein generated by photoinhibition of photosystem II

When photosystem (PS) II-enriched membranes are exposed to strong light, cross-linking of the intrinsic D1 protein with the surrounding polypeptides and degradation of the D1 protein take place. The cross-linking of the D1 protein with the alpha-subunit of cytochrome b(559) is suggested to be an early event of photoinduced damage to the D1 protein (Barbato et al., FEBS Lett. 309 (1992) 165-169). The relationship between the cross-linking and the degradation of the D1 protein, however, is not yet clear. In the present study, we show that the addition of stromal extract from chloroplasts degrades the 41 kDa cross-linked product of D1/cytochrome b(559) alpha-subunit and enhances the degradation of the D1 protein. Incubation of the preilluminated PS II-enriched membranes with the stromal extract at 25 degrees C causes the degradation of the cross-linked product by more than 70%. The activity of the stromal extract showed a pH optimum at 8.0, and was enhanced by the addition of ATP or GTP. Consistent with the nucleotide effect, this stromal activity was eliminated by the preincubation of the stromal extract with apyrase, which hydrolyzes nucleotides. Also, the stromal activity was nearly fully inhibited by a serine-type protease inhibitor, 3,4-dichloroisocoumarin, which suggests participation of a serine-type protease(s).     

Structural conversion between open and closed forms of radixin: low-angle shadowing electron microscopy

The function of ERM (ezrin/radixin/moesin) proteins as general cross-linkers between actin filaments and plasma membranes is regulated downstream of Rho, through the transition between active and inactive forms. To directly examine the conformational change between the active and inactive forms of ERM proteins, we applied low-angle rotary-shadowing electron microscopy to the radixin molecules, wild-type, T564A-non-phosphorylated-type, and T564E-phosphorylated-type, since most of the active forms are reportedly stabilized in cells by the C-terminal threonine phosphorylation. As a result, the T564A- and wild-type radixin molecules yielded the globular closed forms, approximately 8-14 nm in diameter, with some striations on their surfaces. In contrast, the T564E-radixin molecules tended to take elongated open forms, in which two globular structures measuring approximately 8 nm and approximately 5 nm in diameter were associated with both ends of the filamentous structures. The filamentous structure took either a approximately 20-25 nm-long straight course or a folded course. Taken together with the biochemical and the crystal structural results obtained to date, the closed and open forms represent the inactive and active forms of radixin as cross-linkers between actin filaments and plasma membranes.     

Mutation of host delta9 fatty acid desaturase inhibits brome mosaic virus RNA replication between template recognition and RNA synthesis

All positive-strand RNA viruses assemble their RNA replication complexes on intracellular membranes. Brome mosaic virus (BMV) replicates its RNA in endoplasmic reticulum (ER)-associated complexes in plant cells and the yeast Saccharomyces cerevisiae. BMV encodes RNA replication factors 1a, with domains implicated in RNA capping and helicase functions, and 2a, with a central polymerase-like domain. Factor 1a interacts independently with the ER membrane, viral RNA templates, and factor 2a to form RNA replication complexes on the perinuclear ER. We show that BMV RNA replication is severely inhibited by a mutation in OLE1, an essential yeast chromosomal gene encoding delta9 fatty acid desaturase, an integral ER membrane protein and the first enzyme in unsaturated fatty acid synthesis. OLE1 deletion and medium supplementation show that BMV RNA replication requires unsaturated fatty acids, not the Ole1 protein, and that viral RNA replication is much more sensitive than yeast growth to reduced unsaturated fatty acid levels. In ole1 mutant yeast, 1a still becomes membrane associated, recruits 2a to the membrane, and recognizes and stabilizes viral RNA templates normally. However, RNA replication is blocked prior to initiation of negative-strand RNA synthesis. The results show that viral RNA synthesis is highly sensitive to lipid composition and suggest that proper membrane fluidity or plasticity is essential for an early step in RNA replication. The strong unsaturated fatty acid dependence also demonstrates that modulating fatty acid balance can be an effective antiviral strategy.