Untersuchungen zur Virusätiologie einer mit Hepatitis und Befiederungsstörungen einhergehender Krankheit bei Wellensittich-Nestlingen (Melopsittacus undulatus)

Characterization of a calcium binding domain in the VP1 protein of the avian polyomavirus, budgerigar fledgling disease virus

Calcium ions appear to play a major role in maintaining the structural integrity and assembly of papovavirus virions and are likely involved in the process of viral uncoating. Recently it was reported that the purified recombinant VP1 protein of budgerigar fledgling disease virus (BFDV) was capable of assembling into capsid-like particles in the presence of calcium. It is now reported that the major capsid protein VP1 of BFDV binds calcium ions in an in vitro calcium binding assay. Two deletions were made in the VP1 protein to identify a calcium binding domain and to further characterize the role of calcium ions in the capsid assembly process. Recombinant VP1 lacking a putative calcium binding domain (Asp-237-Asp-248) failed to bind radioactive 45Ca2+ yet associated into capsomeres. These capsomeres were similar in shape to the wild-type VP1 but were unable to assemble into capsid-like particles. Likewise, recombinant VP1 lacking ten carboxyl terminal amino acids (Glu-334-Arg-343) also formed capsomeres that were unable to assemble into capsid-like particles. In contrast to the VP1 protein with the internal deletion, the protein with the truncated carboxyl terminus bound 45Ca2+ in the in vitro assay. These results have identified a calcium binding domain (Asp-237-Asp-248) for the BFDV VP1 protein and a crucial role for the VP1 carboxyl terminal amino acids (Glu-334-Arg-343) in capsid assembly.

Purification of recombinant budgerigar fledgling disease virus VP1 capsid protein and its ability for in vitro capsid assembly

A recombinant system for the major capsid VP1 protein of budgerigar fledgling disease virus has been established. The VP1 gene was inserted into a truncated form of the pFlag-1 vector and expressed in Escherichia coli. The budgerigar fledgling disease virus VP1 protein was purified to near homogeneity by immunoaffinity chromatography. Fractions containing highly purified VP1 were pooled and found to constitute 3.3% of the original E. coli-expressed VP1 protein. Electron microscopy revealed that the VP1 protein was isolated as pentameric capsomeres. Electron microscopy also revealed that capsid-like particles were formed in vitro from purified VP1 capsomeres with the addition of Ca2+ ions and the removal of chelating and reducing agents.

Phosphorylation of the budgerigar fledgling disease virus major capsid protein VP1

The structural proteins of the budgerigar fledgling disease virus, the first known nonmammalian polyomavirus, were analyzed by isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The major capsid protein VP1 was found to be composed of at least five distinct species having isoelectric points ranging from pH 6.45 to 5.85. By analogy with the murine polyomavirus, these species apparently result from different modifications of an initial translation product. Primary chicken embryo cells were infected in the presence of 32Pi to determine whether the virus structural proteins were modified by phosphorylation. SDS-PAGE of the purified virus structural proteins demonstrated that VP1 (along with both minor capsid proteins) was phosphorylated. Two-dimensional analysis of the radiolabeled virus showed phosphorylation of only the two most acidic isoelectric species of VP1, indicating that this posttranslational modification contributes to VP1 species heterogeneity. Phosphoamino acid analysis of 32P-labeled VP1 revealed that phosphoserine is the only phosphoamino acid present in the VP1 protein.

Polyomavirus infection in budgerigars (Melopsittacus undulatus): clinical and aetiological studies

In order to get insight into the aetiology of French Moult (FM) and Budgerigar Fledgling Disease (BFD), and to determine relationships between the two diseases, 298 budgerigars from 49 different breeding flocks were examined. From all birds with BFD and from several birds with FM, viruses were isolated which produced characteristic cytopathic changes in chicken embryo fibroblasts. They were insensitive to chloroform treatment, and their replication was inhibited in the presence of 5-iododeoxyuridine. One of these isolates, from a bird exhibiting clinical signs of BFD, was determined by biochemical and biophysical methods to be a polyomavirus (BFDV). Nestling budgerigars 3 to 10 days of age, were inoculated with this BFDV isolate. They developed an acute systemic disease with high mortality rates, similar to naturally occurring infections. In this regard, BFDV differs markedly from all the other polyomaviruses known until now which only cause clinically asymptomatic, persistent infections in their natural hosts. Most of the birds examined with clinical signs of BFD or FM exhibited low titers of antibodies neutralizing the BFDV isolate, whereas in clinically healthy birds from flocks that never had any problems with FM or BFD, no antibodies against BFDV could be detected. On account of the results of our experiments described here we suspect that FM might be a milder, more protracted form of a BFDV infection.

The genome of budgerigar fledgling disease virus, an avian polyomavirus

Budgerigar fledgling disease virus (BFDV) represents the first avian member of the Polyomavirus family. In contrast to mammalian polyomaviruses BFDV exhibits unique biological properties, in particular it is able to cause an acute disease with distinct organ manifestations in affected birds. Here we present the complete nucleotide sequence of the BFDV genome, consisting of 4980 bp. When compared to published nucleotide sequences of other polyomaviruses, the BFDV genome exposes a number of very similar structural features, and undoubtedly qualifies as a member of that family of viruses. The most important differences include a large T antigen remarkably reduced in size, and an origin of replication region with fundamental deviations from the origin structure of all other polyomaviruses. The specific characteristics of the BFDV genome may be used to place this virus into a distinct subgroup within the Polyomavirus family and may give a clue to the elucidation of its extraordinary biological properties.

Cloning and characterization of budgerigar fledgling disease virus, an avian polyomavirus

The DNA of a virus isolated from fledgling budgerigars, designated BFDV, was cloned and analyzed with regard to its relationship to the polyomavirus subgroup of the papovavirus family. Under relaxed conditions, the DNA of BFDV cross-hybridized with the DNAs of members of the polyomavirus subgroup, such as the mouse polyomavirus, the monkey viruses simian virus 40, stump-tailed macaque virus, and lymphotropic papovavirus, and the human viruses JCV and BKV. Under stringent conditions, however, no homologies could be detected. Furthermore, BFDV propagated in chicken embryo cells was antigenically related to the capsid antigen(s) of the other polyomaviruses. The virus was able to transform hamster embryo cells in vitro which is a typical feature of polyomaviruses. These data clearly indicate that BFDV is a new distinct member of the polyomavirus genus representing the first nonmammalian polyomavirus.