Identification of a PDZ protein, PIST, as a binding partner for Rho effector Rhotekin: biochemical and cell-biological characterization of Rhotekin-PIST interaction

Among various effector proteins for the small GTPase Rho, the function(s) of Rhotekin is (are) almost unknown. We have identified PIST [PDZ (PSD-95, Discs-large and ZO-1) domain protein interacting specifically with TC10 (a Rho-family small GTPase)] as a binding partner for Rhotekin, using yeast two-hybrid screening. Rhotekin was found to associate with PIST in vitro and in both polarized and non-polarized MDCK (Madin-Darby canine kidney) cells. The C-terminal SPV (Ser-Pro-Val) motif of Rhotekin exhibited binding to the PDZ domain of PIST. The binding was markedly inhibited by an activated version of Rho and partially by that of Rac or Cdc42 in COS7 cells. In contrast, TC10 had no effects on the binding. Immunofluorescence analyses revealed the co-localization of PIST and Rhotekin at the Golgi apparatus in non-polarized fibroblast-like MDCK cells and AJs (adherens junctions) in the fully polarized cells. PIST and Rhotekin are recruited from the cytosol to AJs as the cell becomes polarized. Expression of constitutively active Rho or prevention of Rhotekin-PIST interaction induced diffuse cytoplasmic distribution of Rhotekin in polarized MDCK cells. These results suggest that there is (1) Rho-dependent regulation of Rhotekin-PIST interaction, (2) involvement of PIST in the recruitment of Rhotekin to AJs and (3) a possible role(s) for these two proteins in cell-polarity development and/or maintenance.     

The plasma membrane shuttling of CAPRI is related to regulation of mast cell activation

The Ca(2+)-promoted Ras inactivator (CAPRI), a Ras GTPase-activating protein, is involved in the inactivation of mitogen-activated protein kinase pathway. However, a precise role of CAPRI in immune responses is still unknown. Here we showed that overexpression of CAPRI suppresses antigen-induced degranulation and cytokine production in mast cells (RBL cells). Antigen elicited the translocation of CAPRI to the plasma membrane from the cytoplasm, which was concomitant with the increase in the intracellular Ca(2+) concentration. The nuclear import of extracellular signal-regulated kinase 2 (ERK2) occurred after the re-localization of CAPRI to the cytoplasm in the mast cells, suggesting that the early phase of ERK2 activation is eliminated. A mutant of GAP-related domain, CAPRI(R472S), showed a feeble translocation to the plasma membrane but did not affect the degranulation, ERK2 activation, and cytokine production. The results suggested that the translocation of CAPRI to the plasma membranes regulates crucially cellular responses in mast cells.     

Identification and characterisation of a leucine aminopeptidase from the hard tick Haemaphysalis longicornis

Aminopeptidases responsible for blood digestion have yet to be identified in haematophagous ticks. We report here the cloning and molecular characterisation of a cDNA encoding leucine aminopeptidase, a member of the M17 cytosolic aminopeptidase family, from the hard tick Haemaphysalis longicornis (HlLAP). Endogenous HlLAP was detected in the soluble fraction of adult tick extracts by immunoblotting. Immunohistochemical studies demonstrated that endogenous HlLAP expression mainly took place in the cytosol of midgut epithelial cells. Furthermore, expression of HlLAP was induced by a blood-feeding process. A functional recombinant HlLAP expressed in Escherichia coli efficiently hydrolyses synthetic substrates for aminopeptidase, a leucyl (with the Km value 0.19 +/- 0.011 mM and Vmax value 157.2 +/- 3.17 nmol/min/mgprotein) and a methionyl substrate (with the Km value 0.12+/-0.0052 mM and Vmax value 171.9 +/- 2.31 nmol/min/mgprotein). Enzyme activity was found to be optimum at pH 8 and 35 degrees C. The recombinant HlLAP enzyme activity was strongly dependent on metal divalent cations, Mn2+, and was inhibited by bestatin. These results indicate that HlLAP play an important role for host's blood digestion process.     

Role of the carboxy-terminal region of the outer membrane protein AatA in the export of dispersin from enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC) is an emerging enteric pathogen in both developing and industrialized countries. AatA, an outer-membrane protein that is a homolog of E. coli TolC, facilitates the export of the dispersin protein Aap across the outer membrane in EAEC. To identify which amino acids are important for this export activity, site-directed mutagenesis of the carboxy terminus was performed. An insertional mutant of aatA was complemented with each of several deletion mutants, and was examined for Aap secretion. The results showed that three nonpolar amino acids at positions 381-383 (Phe-Leu-Leu) were required for the activity, and these residues were located at the base of carboxy-terminal elongation in the equatorial domain of AatA.     

A secreted cystatin from the tick Haemaphysalis longicornis and its distinct expression patterns in relation to innate immunity

Proteins capable of selective and specific inhibition of cysteine protease have been identified as cystatins and are isolated from a variety of microbes and tissues of animals and plants. The physiological function of these proteins has been proposed to be the regulation of protein turnover and defense against pathogens as well as the balance of the host-parasite immune relationship. Genes encoding cystatins have been found in several species of ticks, but the function of cystatin in ticks is not understood. We cloned a gene encoding cystatin from tick H. longicornis and designated it as Hlcyst-2 (H. longicornis cystatin-2). Its full-length cDNA is 569 bp, and it encodes a putative 133 amino acid protein with an obvious signal peptide. Sequence analysis demonstrated that it has significant homology with the known cystatin. The recombinant protein was expressed in a GST-fused soluble form in Escherichia coli and purified by affinity chromatography. The inhibitory activity of the recombinant protein against papain, cathepsin L, and cathepsin B was identified by fluorogenic substrate analysis. Cystatin was mostly expressed in the tick midgut and hemocyte. Blood feeding induced significantly increased expression in the midgut. Real-time PCR confirmed that LPS-injected adult ticks expressed Hlcyst-2 1.6 more times than the PBS-injected control; Babesia gibsoni-infected larvae ticks expressed Hlcyst-2 1.8 more times than normal larvae ticks. The recombinant protein also showed a significant growth-inhibitory effect on Babesia bovis cultured in vitro. These results indicated this cystatin Hlcyst-2 is involved in tick innate immunity.     

Physical and functional interaction between DDB and XPA in nucleotide excision repair

Damaged DNA-binding protein (DDB), consisting of DDB1 and DDB2 subunits recognizes a wide spectrum of DNA lesions. DDB is dispensable for in vitro nucleotide excision repair (NER) reaction, but stimulates this reaction especially for cyclobutane pyrimidine dimer (CPD). Here we show that DDB directly interacts with XPA, one of core NER factors, mainly through DDB2 subunit and the amino-acid residues between 185 and 226 in XPA are important for the interaction. Interestingly, the point mutation causing the substitution from Arg-207 to Gly, which was previously identified in a XP-A revertant cell-line XP129, diminished the interaction with DDB in vitro and in vivo. In a defined system containing R207G mutant XPA and other core NER factors, DDB failed to stimulate the excision of CPD, although the mutant XPA was competent for the basal NER reaction. Moreover, in vivo experiments revealed that the mutant XPA is recruited to damaged DNA sites with much less efficiency compared with wild-type XPA and fails to support the enhancement of CPD repair by ectopic expression of DDB2 in SV40-transformed human cells. These results suggest that the physical interaction between DDB and XPA plays an important role in the DDB-mediated NER reaction.     

VLA-5-mediated Adhesion to Fibronectin Accelerates Hemin-stimulated Erythroid Differentiation of K562 Cells through Induction of VLA-4 Expression

Fibronectin plays important roles in erythropoiesis through the fibronectin receptors VLA-4 and VLA-5. However, the substantial role of these fibronectin receptors and their functional assignment in erythroid differentiation are not yet fully understood. Here, we investigated the effects of cell adhesion to fibronectin on erythroid differentiation using K562 human erythroid progenitor cells. Erythroid differentiation could be induced in K562 cells in suspension by stimulating with hemin. This hemin-stimulated erythroid differentiation was highly accelerated when cells were induced to adhere to fibronectin by treatment with TNIIIA2, a peptide derived from tenascin-C, which has recently been found to induce β1-integrin activation. Another integrin activator, Mn²⁺, also accelerated hemin-stimulated erythroid differentiation. Adhesive interaction with fibronectin via VLA-4 as well as VLA-5 was responsible for acceleration of the hemin-stimulated erythroid differentiation in response to TNIIIA2, although K562 cells should have been lacking in VLA-4. Adhesion to fibronectin forced by TNIIIA2 causally induced VLA-4 expression in K562 cells, and this was blocked by the RGD peptide, an antagonist for VLA-5. The resulting adhesive interaction with fibronectin via VLA-4 strongly enhanced the hemin-stimulated activation of p38 mitogen-activated protein kinase, which was shown to serve as a signaling molecule crucial for erythroid differentiation. Suppression of VLA-4 expression by RNA interference abrogated acceleration of hemin-stimulated erythroid differentiation in response to TNIIIA2. Thus, VLA-4 and VLA-5 may contribute to erythropoiesis at different stages of erythroid differentiation.Hematopoietic stem and progenitor cells proliferate and differentiate in the bone marrow and fetal liver (1–6). Stromal cells of the bone marrow and fetal liver form a hematopoietic microenvironment called a “niche.” This microenvironment niche plays a crucial role in the regulation of the proliferation and differentiation of hematopoietic stem and progenitor cells. Besides humoral factors that include hematopoietic growth factors, adhesive interaction of hematopoietic stem and progenitor cells with stromal cells and/or the extracellular matrix (ECM)² in the hematopoietic microenvironment is indispensable for hematopoietic development (1–6). The ECM in the hematopoietic microenvironment is composed of various macromolecules, such as fibronectin (FN), collagens, laminins, and proteoglycans. Among them, FN is one of the most important parts of the microenvironment niche (7–11). Also, in erythropoiesis, the importance of the adhesion of erythroid progenitors to FN via the FN receptors VLA-4 and VLA-5 has been reported (11–16). However, the substantial role of these FN receptors and their functional assignment in erythroid differentiation are not yet fully understood.We previously found that FN, which provides scaffolding for the adhesion of various cell types, has an alternative functional site opposing cell adhesion (17). A 22-mer peptide derived from the 14th FN type III-like (FNIII) repeat of the FN molecule, termed FNIII14, strongly suppresses cell adhesion to FN by inhibiting the activation of β1-integrins including VLA-4 and VLA-5 (18, 19). Conversely, we have recently found that tenascin (TN)-C, which is an anti-adhesive ECM protein (20, 21), has a functional site for stimulating cell adhesion to FN (22). A 22-mer peptide derived from the FNIII repeat A2 in the TN-C molecule, termed TNIIIA2, can induce the conformational change necessary for functional activation of FN receptors through binding with syndecan-4 (22, 23). The active sites of FNIII14 and TNIIIA2 appear to be cryptic in the molecular structures of FN and TN-C but are exposed by conformational change through interaction with other ECM molecules or by processing with matrix metalloproteinase-2 (22, 24). Thus, these functional sites found in FN and TN-C molecules, which act in opposition to their parental ECM proteins, may act as a negative feedback loop for preventing excessive cellular responses to these ECM proteins in biological processes with ECM rearrangement. In any case, FNIII14 and TNIIIA2 enable us to control, either negatively or positively, the adhesion of various cell types to FN.Various hematopoietic progenitor cell lines have been used in in vitro studies of hematopoietic differentiation. However, most hematopoietic progenitor cell lines are nonadherent, because their cell surface β1-integrins, including FN receptors, have impaired ligand-binding activity (25, 26). Therefore, in order to investigate the role of cell adhesion to FN in hematopoietic differentiation, their FN receptors must be activated. Since TNIIIA2 can induce activation of FN receptors in various hematopoietic progenitor cell lines (22), this peptide factor may be useful for investigating the substantial role of cell adhesion to FN in hematopoietic differentiation. Here, we investigate the effects of cell adhesion to FN on erythroid differentiation using TNIIIA2 and Mn²⁺ as the integrin activator and the human erythroid progenitor cell line K562, which only expresses VLA-5, as the FN receptor (27). As a result, we show that hemin-stimulated erythroid differentiation of K562 cells is strongly enhanced when K562 cells are forced to adhere to FN. Sustained adhesion to FN via VLA-5, which is induced by TNIIIA2 or Mn²⁺, causes induction of VLA-4 expression. The resulting adhesive interaction with FN via newly expressed VLA-4 then generates a conspicuous increase in the hemin-stimulated phosphorylation/activation of p38 MAP kinase, which is shown to serve as a signaling molecule crucial for erythroid differentiation of K562 cells.     

Establishment and characterization of the Bombyx mandarina cell line

A new cell line, designated as NIAS-Boma-529b, was established from the larval fat bodies of Bombyx mandarina (B. mandarina), which is believed to be an ancestor of Bombyx mori (B. mori). This cell line has been cultured for approximately 150 passages during 2 years in an IPL-41 medium supplemented with 10% fetal bovine serum at a constant temperature of 26 °C. The morphology of this line includes adhesive round and spindle-shaped cells. Random-amplified polymorphic DNA analysis (RAPD) using 7 primers and a statistical analysis based on Nei’s genetic distance revealed that this cell line was closely related to B. mori-derived cell lines. An infection study also revealed that this cell line was susceptible to B. mori nucleopolyhedrovirus (BmNPV); however, it had no apparent susceptibility to Autographa californica NPV (AcNPV), which is closely related to BmNPV. Nevertheless, cells infected with AcNPV showed an extensive cytopathic effect (CPE), including a rough cell surface, rounding, nuclear expansion, and cell blebbing. These results suggest that this cell line can be useful to clarify the mechanism of host range determination of BmNPV and AcNPV.