Regulation of phospholipase C-gamma(1) by the actin-regulatory protein villin

The actin-regulatory protein villin is tyrosinephosphorylated and associates with phospholipase C-₁(PLC-₁) in the brush border of intestinalepithelial cells. To study the mechanism of villin-associatedPLC-₁ activation, we reconstituted in vitro the tyrosinephosphorylation of villin and its association with PLC-₁. Recombinant villin was phosphorylated in vitro bythe nonreceptor tyrosine kinase c-src or by expression in the TKX1 competent cells that carry an inducible tyrosine kinase gene. Using invitro binding assays, we demonstrated that tyrosine-phosphorylated villin associates with the COOH-terminal Src homology 2 (SH2) domain ofPLC-₁. The catalytic activity of PLC-₁was inhibited by villin in a dose-dependent manner with half-maximalinhibition at a concentration of 12.4 µM. Villin inhibitedPLC-₁ activity by sequestering the substratephosphatidylinositol 4,5-bisphosphate (PIP₂), sinceincreasing concentrations of PIP₂ reversed the inhibitory effects of villin on PLC activity. The inhibition ofPLC-₁ activity by villin was reversed by the tyrosinephosphorylation of villin. Further, we demonstrated that tyrosinephosphorylation of villin abolished villin's ability to associate withPIP₂. In conclusion, tyrosine-phosphorylated villinassociates with the COOH-terminal SH2 domain of PLC-₁and activates PLC-₁ catalytic activity. Villin regulatesPLC-₁ activity by modifying its own ability to bindPIP₂. This study provides biochemical proof of thefunctional relevance of tyrosine phosphorylation of villin andidentifies the molecular mechanisms involved in the activation ofPLC-₁ by villin.

     

A new method for protein coexpression in Escherichia coli using two incompatible plasmids

It is commonly believed that incompatible plasmids carrying the same replicon cannot coexist stably in one Escherichia coli cell. However, we found that two incompatible plasmids carrying different antibiotic resistance genes, if under the selection pressure of the two antibiotics, can coexist in E. coli for at least 14 h, which is adequate for routine culture and protein expression. Based on this discovery, we developed a new method to coexpress foreign proteins in E. coli using two incompatible plasmids. The coding regions of the two subunits (DFF45 and DFF40) of the human DNA fragmentation factor (DFF) were cloned into two incompatible bacterial expression vectors-pET-21a with ampicillin resistance and pET-28a with kanamycin resistance, respectively. The two resulting plasmids were used to cotransform E. coli BL21(DE3) cells. After selection by ampicillin and kanamycin simultaneously, cotransformants that contain both recombinant plasmids were obtained. Induced by isopropyl beta-d-thiogalactoside, DFF45, and DFF40 were coexpressed efficiently in the presence of the two antibiotics. The coexpression product contained adequate soluble portions for both DFF45 and DFF40, while all DFF40 was insoluble if expressed alone. The coexpression product also exhibited the same caspase-activated DNase activity as its natural counterparts, which cannot be obtained if its two subunits are expressed separately.     

Crystal structure of GGA2 VHS domain and its implication in plasticity in the ligand binding pocket

Golgi-localized, gamma-ear-containing, ARF binding (GGA) proteins regulate intracellular vesicle transport by recognizing sorting signals on the cargo surface in the initial step of the budding process. The VHS (VPS27, Hrs, and STAM) domain of GGA binds with the signal peptides. Here, a crystal structure of the VHS domain of GGA2 is reported at 2.2 A resolution, which permits a direct comparison with that of homologous proteins, GGA1 and GGA3. Significant structural difference is present in the loop between helices 6 and 7, which forms part of the ligand binding pocket. Intrinsic fluorescence spectroscopic study indicates that this loop undergoes a conformational change upon ligand binding. Thus, the current structure suggests that a conformational change induced by ligand binding occurs in this part of the ligand pocket.     

Analysis of the 3' Cmu region of the rabbit Ig heavy chain locus

The immunoglobulin D (IgD) antibody class was, for many years, identified only in primates, rodents and teleost fish. The limited distribution of IgD among vertebrates suggested that IgD is a functionally redundant antibody class that has been lost by many vertebrate species during evolution. The recent identification of IgD in artiodactyls, however, suggests that IgD might be more widely expressed among vertebrates than previously thought, possibly serving a unique role in immunity. IgD expression has been searched for but not detected in rabbits. In order to search directly for a rabbit Cdelta locus encoding the constant region of IgD, we determined the nucleotide sequence of 13.5 kb of genomic DNA downstream of the rabbit Cmu locus. We did not find a rabbit Cdelta locus in this region, but found instead that this region is densely populated by repetitive elements, including a long interspersed DNA element repeat, six C repeats, and two processed pseudogenes. We conclude that the rabbit probably does not express IgD because there is no Cdelta locus immediately downstream of the rabbit Cmu locus.     

The CD154-CD40 T cell costimulation pathway is required for host sensitization of CD8(+) T cells by skin grafts via direct antigen presentation

Although the CD154-CD40 T cell costimulation pathway has been shown to mediate alloimmune responses in normal recipients, little is known about its role in sensitized hosts. In this work, by using novel models of cardiac allograft rejection in skin-sensitized CD154- and CD40-deficient mice, we reaffirm the key role of CD154-CD40 signaling in host sensitization to alloantigen in vivo. First, we identified CD8(+) T cells as principal effectors in executing accelerated rejection in our model. Disruption of CD154-CD40 signaling in recipients at the T cell side (CD154-deficient) but not at the APC side (CD40-deficient) abrogated accelerated (<2 days) rejection and resulted in long-term (>100 days) graft survival. This suggests that the CD154-dependent mechanism in host CD8(+) T cell sensitization operates via the direct Ag presentation. Then, in comparative studies of alloimmune responses in CD154-deficient and wild-type recipients, we showed that, although alloreactive B cell responses were inhibited, alloreactive T cell responses were down-regulated selectively in the CD8(+) T cell compartment, leaving CD4(+) T cells largely unaffected. This unique alteration in host alloreactivity, seen not only in peripheral lymphocytes but also in allograft infiltrate, may represent the key mechanism by which disruption of CD154-CD40 signaling prevents sensitization to alloantigen in vivo and leads to long-term allograft survival.     

Phosphatidylserine binding alters the conformation and specifically enhances the cofactor activity of bovine factor Va

Factor V(a) is a cofactor for the serine protease factor X(a) that activates prothrombin to thrombin in the presence of Ca(2+) and a platelet membrane surface. A platelet membrane lipid, phosphatidylserine (PS), regulates the proteolytic activity of factor X(a) as well as the structure of prothrombin. Here we ask whether PS also regulates the structure and cofactor activity of factor V(a), which is a heterodimer composed of one heavy chain (A1-A2 domains) and one light chain (A3-C1-C2 domains). We use fluorescence, circular dichroism, equilibrium dialysis, and activity measurements to demonstrate the following: (1) Factor V(a) has four sites for dicaproyl-sn-glycero-3-phospho-L-serine (C(6)PS, a soluble form of PS); the heavy and light chains each bind two C(6)PS molecules. (2) In the absence of Ca(2+), only two sites remain, one in the heavy chain and another in the light chain. (3) Binding to these sites causes conformational changes evidenced by changes in intrinsic fluorescence and in CD spectra and changes in cofactor activity. (4) At least some of the four lipid binding sites are nonspecific with respect to soluble lipid species, but the site(s) that regulate(s) cofactor activity is (are) specific for C(6)PS, phosphatidic acid, or phosphatidyl(homo)serine and produce a response comparable to that seen with a PS-containing membrane. (5) Like Ca(2+), C(6)PS also mediates the interaction between factor V(a) heavy (V(a)-HC) and light (V(a)-LC) chains. We conclude that PS regulates both the cofactor and the enzyme of the prothrombin-activating complex.     

Regulation of actin dynamics by tyrosine phosphorylation: identification of tyrosine phosphorylation sites within the actin-severing domain of villin

We have previously shown that villin, an epithelial cell actin-binding protein, is tyrosine phosphorylated both in vitro and in vivo and that villin's actin-modifying functions are regulated by phosphorylation. Here as a first step toward understanding the role of villin tyrosine phosphorylation, we sought to identify the major phosphorylation site(s) in human villin and study its role in actin filament assembly. We generated a series of carboxyl-terminal truncation mutants of villin and cloned them in the prokaryotic expression vector pGEX-2T. Full-length villin and the truncation mutants were expressed in TKX1 cells, which carry an inducible tyrosine kinase gene. Using this approach, we identified a region in the amino-terminal actin-severing domain of villin as the site of phosphorylation (amino acids 1-261). Five phosphorylation sites were identified by direct mutation of candidate tyrosines (Y) to phenylalanine (F), namely, Y46, -60, -64, -81, and -256. Changing all of these sites to phenylalanine resulted in a villin mutant that neither was phosphorylated in TKX1 cells nor was a substrate for c-src kinase in an in vitro kinase assay. Using a pyrene actin-based fluorescence assay, we mapped the various phosphorylated tyrosine residues with the actin-nucleating and -depolymerizing functions of villin. Phosphorylation of any one of the identified sites inhibited the actin-nucleating function of villin, whereas phosphorylation at Y46 and/or Y60 increased the actin-severing activity of villin. Since there is significant homology between the amino-terminal end of villin and other actin-severing proteins, the results provide a structural basis for the actin-severing mechanism and help understand the relationship of phosphorylation with this function.     

Enhanced activity of Ca2+-activated K+ channels by 1-[2-hydroxy-3-propyl-4-[(1H-tetrazol-5-yl)butoxyl]phenyl] ethanone (LY-171883) in neuroendocrine and neuroblastoma cell lines

The effects of LY-171883, an orally active leukotriene antagonist, on membrane currents were examined in pituitary GH(3) and in neuroblastoma IMR-32 cells. In GH(3) cells, LY-171883 (1-300 microM) reversibly increased the amplitude of Ca(2+)-activated K(+) current in a concentration-dependent manner with an EC(50) value of 15 microM. In excised inside-out patches recorded from GH(3) cells, the application of LY-171883 into cytosolic face did not modify single channel conductance of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels; however, it did increase the channel activity. The LY-171883-stimulated activity of BK(Ca) channels is dependent on membrane potential, and results mainly from an increase in mean open time and a decrease in mean closed time. However, REV-5901 (30 microM) suppressed the activity of BK(Ca) channels and MK-571 (30 microM) did not have any effect on it. Under the current-clamp condition, LY-171883 (30 microM) caused membrane hyperpolarization as well as decreased the firing rate of action potentials in GH(3) cells. In neuroblastoma IMR-32 cells, the application of LY-171883 (30 microM) also stimulated BK(Ca) channel activity in a voltage-dependent manner. However, neither clofibrate (30 microM) nor leukotriene D(4) (10 microM) affected the channel activity in IMR-32 cells. Troglitazone (30 microM) decreased the channel activity, but ciglitazone (30 microM) enhanced it. This study clearly demonstrates that LY-171883 stimulates the activity of BK(Ca) channels in a manner unlikely to be linked to its blockade of leukotriene receptors or stimulation of peroxisome proliferator-activated receptors. The stimulatory effects on these channels may, at least in part, contribute to the underlying cellular mechanisms by which LY-171883 affects neuronal or neuroendocrine function.