Rapid Selection in Modified BHK-21 Cells of a Foot-and-Mouth Disease Virus Variant Showing Alterations in Cell Tropism

With persistent foot-and-mouth disease virus (FMDV) in BHK-21 cells, there is coevolution of the cells and the resident virus; the virulence of the virus for the parental BHK-21 cells is gradually increased, and the cells become partially resistant to FMDV. Here we report that variants of FMDV C₃Arg/85 were selected in a single infection of partially resistant BHK-21 cells (termed BHK-Rb cells). Indirect immunofluorescence showed that the BHK-Rb cell population was heterogeneous with regard to susceptibility to C₃Arg/85 infection. Infection of BHK-Rb cells with C₃Arg/85 resulted in an early phase of partial cytopathology which was followed at 6 to 10 days postinfection by the shedding of mutant FMDVs, termed C₃-Rb. The selected C₃-Rb variants showed increased virulence for BHK-21 cells, were able to overcome the resistance of modified BHK-21 cells to infection, and had acquired the ability to bind heparin and to infect wild-type Chinese hamster ovary (CHO) cells. A comparison of the genomic sequences of the parental and modified viruses revealed only two amino acid differences, located at the surface of the particle, at the fivefold axis of the viral capsid (Asp-9→Ala in VP3 and either Gly-110→Arg or His-108→Arg in VP1). The same phenotypic and genotypic modifications occurred in a highly reproducible manner; they were seen in a number of independent infections of BHK-Rb cells with viral preparation C₃Arg/85 or with clones derived from it. Neither amino acid substitutions in other structural or nonstructural proteins nor nucleotide substitutions in regulatory regions were found. These results prove that infection of partially permissive cells can promote the rapid selection of virus variants that show alterations in cell tropism and are highly virulent for the same cells.     

Topography of a flexible ribonucleoprotein helix: protein-protein contacts in Sendai virus nucleocapsids.

Contacts among the three polypeptide species in the flexible helical nucleocapsids of a paramyxovirus were examined with bifunctional protein cross-linking reagents. Polypeptides L and P, minor components of Sendai virus nucleocapsids implicated in viral RNA polymerase activity, were efficiently cross-linked into large complexes, indicating that they enjoy abundant contacts with neighboring protein molecules in the helix. Less reactivity was found in the case of the major structural polypeptide, NP; about half of all molecules of NP formed large cross-linked complexes, most of the rest remaining as monomers along with a small proportion of homodimers and low-order oligomers. Marked heterogeneity in the cross-linking reactivity of NP molecules, which may reflect the conformational quasi-equivalence inherent in a flexible helix, was indicated by the production of several conformers of homodimers and other low-order oligomers of NP, and by failure of the kinetics of NP cross-linking to conform to a simple statistical model of random polmerization. The validity of the statistical model was shown by cross-linking experiments with the rigid helical virus, tobacco mosaic virus.     

Identification of Sendai virus mRNA species.

Cell-free translation of separated Sendai virus mRNA species identified the message for polypeptide M and suggested the identity of the message for polypeptide NP.     

Isolation and characterization of epithelial cells from bovine pancreatic duct

Epithelial cells derived from bovine pancreatic duct have been grown continuously in culture for 30 weeks (approximately 90 doublings of the cell population). The cells were grown in Eagle's minimal essential medium supplemented with 10% heat-inactivated fetal bovine serum, 2 mM glutamine, 0.1 mM nonessential amino acids, and antibiotics. In confluent cultures, the cells are multilayered and form circular structures. When tested at various passages, the cells neither formed colonies in soft agar nor produced tumors after inoculation into athymic, nude mice. Hydrocortisone (1 and 5 microgram per ml) and insulin (1,5 and 10 microgram per ml) had no effect on the growth of the cells. beta-Retinyl acetate inhibited growth rate and cell yield at a concentration of 5 microgram per ml but was not growth-inhibitory at lower concentrations. By electron microscopy the cells have numerous mitochondria, Golgi and microvilli. Mucous droplets were observed in a small proportion of the cells. Desmosome-like structures and occluding junctions were observed more frequently between cells that had been transferred as aggregates than between cells transferred as single cells. Cytochemical studies indicated that some cells produce PAS positive granules that were not removed after treatment of the cultures with diastase. Eleven cell clones were isolated from the mass culture. The growth rates of the clones are different as well as the period of time in which the clones can be propagated in vitro.     

High-level expression of recombinant human FK-binding protein from a fusion precursor

The human peptidyl-prolyl isomerase FK-binding protein (FKBP) was cloned as a fusion partner with CMP-KDO synthetase (CKS), and the resultant construct was characterized as an improved high-expression source for FKBP. The CKS-FKBP fusion was expressed as a soluble protein at levels approaching 1 gm/L inEscherichia coli fermentations. The fusion protein was purified to near homogeneity by a one-step ammonium sulfate fractionation of whole cell lysate. After selective cleavage, the fusion precursor produced yields approaching 300 mg of purified FKBP per liter of harvested culture, a 30 to 60-fold increase over that observed for a nonfusion construct. Selective cleavage of the fusion partners was accomplished using either hydroxylamine or specific, limited proteolysis. Once separated from the CKS fusion partner, the FKBP was isolated in a single step by either reversed-phase HPLC or chromatography on Q-Sepharose. For comparison of physical and chemical properties, a nonfusion construct of recombinant human FKBP was expressed inE. coli and isolated. The purified FKBPs exhibited expected SDS-PAGE molecular weights and N-terminal sequences. The proteins had similar proton NMR spectra and binding to [³H]FK-506. The fusion construct, CKS-FKBP, was also found to bind [³H]FK-506. These data indicate that FKBP fused to the C-terminus of CKS folds independently of the fusion partner and suggests the fused FKBP adopts a conformation resembling that of the native protein.     

Independence of progesterone blockade of the luteinizing hormone surge in ewes from opioid activity at naloxone-sensitive receptors.

Fifteen ovariectomized ewes were treated with implants (s.c.) creating circulating luteal progesterone concentrations of 1.6 +/- 0.1 ng ml-1 serum. Ten days later, progesterone implants were removed from five ewes which were then infused with saline for 64 h (0.154 mol NaCl l-1, 20 ml h-1, i.v.). Ewes with progesterone implants remaining were infused with saline (n = 5) or naloxone (0.5 mg kg-1 h-1, n = 5) in saline for 64 h. At 36 h of infusion, all ewes were injected with oestradiol (20 micrograms in 1 ml groundnut oil, i.m.). During the first 36 h of infusion, serum luteinizing hormone (LH) concentrations were similar in ewes infused with saline after progesterone withdrawal and ewes infused with naloxone, but with progesterone implants remaining (1.23 +/- 0.11 and 1.28 +/- 0.23 ng ml-1 serum, respectively, mean +/- SEM, P greater than 0.05). These values exceeded circulating LH concentrations during the first 36 h of saline infusion of ewes with progesterone implants remaining (0.59 +/- 0.09 ng ml-1 serum, P less than 0.05). The data suggested that progesterone suppression of tonic LH secretion, before oestradiol injection, was completely antagonized by naloxone. After oestradiol injection, circulating LH concentrations decreased for about 10 h in ewes of all groups. A surge in circulating LH concentrations peaked 24 h after oestradiol injection in ewes infused with saline after progesterone withdrawal (8.16 +/- 3.18 ng LH ml-1 serum).(ABSTRACT TRUNCATED AT 250 WORDS)     

Novel propeptide function in 20 S proteasome assembly influences beta subunit composition

The assembly of eukaryotic 20 S proteasomes involves the formation of half-proteasomes where precursor beta-type subunits gather in position on an alpha-subunit ring, followed by the association of two half-proteasomes and beta-subunit processing. In vertebrates three additional beta-subunits (beta1i/LMP2, beta2i/MECL1, and beta5i/LMP7) can be synthesized and substituted for constitutive homologues (beta1/delta, beta2/Z, and beta5/X) to yield immunoproteasomes, which are important for generating certain antigenic peptides. We have shown previously that when all six beta-subunits are present, cooperative assembly mechanisms limit the diversity of proteasome populations. Specifically, LMP7 is incorporated preferentially over X into preproteasomes containing LMP2 and MECL1. We show here that the LMP7 propeptide is responsible for this preferential incorporation, and it also enables LMP7 to incorporate into proteasomes containing delta and Z. In contrast, the X propeptide restricts incorporation to proteasomes with delta and Z. Furthermore, we demonstrate that the LMP7 propeptide can function in trans when expressed on LMP2, and that its NH(2)-terminal and mid-regions are particularly critical for function. In addition to identifying a novel propeptide function, our results raise the possibility that one consequence of LMP7 incorporation into both immunoproteasomes and delta/Z proteasomes may be to increase the diversity of antigenic peptides that can be generated.