Phosphorylation of the budgerigar fledgling disease virus major capsid protein VP1

Global analysis of modifications of the human BK virus structural proteins by LC-MS/MS

BK virus, a human polyomavirus, may cause nephritis and urological disorders in patients who have undergone renal transplantation. Little is known about the characteristics of the BK viral proteins. In the current study, BK viral proteins were characterized by immunoblotting and LC-MS/MS. The results revealed that BK virus is composed of three structural proteins, VP1, VP2, and VP3 and four cellular histones, H2A, H2B, H3, and H4. The major structural protein, VP1, can be divided into 16 subspecies by two-dimensional gel electrophoresis. Modifications of VP1, VP2, and VP3 were comprehensively identified by LC-MS/MS. The presence of acetylation, cysteinylation, carboxymethylation, carboxyethylation, formylation, methylation, methylthiolation, oxidation, dioxidation, and phosphorylation could be identified. This is the first report providing an analysis of the global modifications present on polyomavirus structural proteins. The identification of these modifications of VP1, VP2, and VP3 should facilitate an understanding of the physiology of BKV during its life cycle.

Agnoprotein-1a of avian polyomavirus budgerigar fledgling disease virus: identification of phosphorylation sites and functional importance in the virus life-cycle

The avian polyomavirus budgerigar fledgling disease virus (BFDV) encodes an unusual set of four agnoproteins in its late upstream region. Of the two pairs of these proteins, which overlap each other in two different reading frames, the p(L1)-promoted agnoprotein-1a (agno-1a) is the dominant species and is able to support virus propagation in the absence of the other three polypeptides. Viral BFDV agno-1a, and also agno-1a expressed via an influenza virus vector, consists of a complex series of electrophoretically separable subspecies that can be reduced by phosphatase action down to a primary unphosphorylated protein with an apparent molecular mass of 31 kDa. Through peptide mass spectrometry and site-directed mutagenesis, the positions of four serine and three threonine residues have been determined as phosphate-accepting groups, which are partially modified by the combined action of three different cellular kinases. Since extensively phosphorylated agno-1a is required for its intracellular function, control over VP protein expression, and unphosphorylated agno-1a is observed as an additional component in the BFDV virion, both extreme subspecies appear to be drawn from that complex mixture, which also includes the intermediate stages of phosphorylation.

Post translational modifications of recombinant human papillomavirus type 6b major capsid protein

We have determined the post-translational modifications of the major capsid protein, L1 of human papillomavirus (HPV) type 6b. Since this virus cannot be cultured in the laboratory to obtain sufficient material for a study, a recombinant L1 protein produced in a vaccinia virus expression system was used in this investigation. Our results show that this protein is phosphorylated at serine residues and is also glycosylated. No myristoylation or palmitoylation was detected. The fraction of L1 protein incorporated into virus-like particles was not glycosylated. Since recombinant L1 protein is a potential human vaccine candidate, knowledge of the post-translation modifications of this protein may prove useful for the design of anti-HPV vaccines.

Polyomavirus major capsid protein VP1 is capable of packaging cellular DNA when expressed in the baculovirus system

Using the p2Bac dual multiple cloning site transfer vector, the polyomavirus major capsid protein gene VP1 was cloned for expression in the baculovirus-insect cell expression system. The 5-day-infected cellular lysate from this recombinant preparation was purified by cesium chloride density gradient centrifugation. Capsid-like particles were observed in the resulting preparation. The purified particle preparation was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was shown to have accurately expressed the polyomavirus VP1 protein as cloned. It was found that the preparation revealed the presence of host histones in the stained gels, which is indicative of DNA packaging. To determine if cellular DNA was being packaged in the particles, Sf9 insect cells were prelabeled with [3H] thymidine. The label was removed, and the cells were subsequently infected with a recombinant Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) carrying the polyomavirus VP1 gene. Upon purification through three cesium chloride gradients and DNase I treatment, capsid-like particles, containing [3H]thymidine-labeled DNA, were isolated which were found to coincide with hemagglutination activity. Studies have indicated that the AcMNPV appears to have the ability to fragment Sf9 cellular DNA. When infected with the recombinant AcMNPV carrying the VP1 gene of polyomavirus, these host DNA fragments are being packaged by the VPI major capsid protein; further, these DNA fragments have been shown to be approximately 5 kb in size, which corresponds to the size of the native polyomavirus genome. These studies demonstrate that the recombinant polyomavirus VP1 protein has the ability to package DNA in the absence of the minor structural proteins VP2 and VP3 and independently of the polyomavirus T antigens.

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.

Suppression of hepatitis B virus expression and replication by hepatitis C virus core protein in HuH-7 cells

Hepatitis B and C viruses (HBV and HCV, respectively) are associated with acute and chronic liver diseases and hepatocellular carcinoma. To elucidate the molecular status of superinfection with these two hepatitis viruses, we cotransfected the full-length or truncated version of HCV structural genes (core and envelope 1) together with the cloned HBV DNA into a human hepatoma cell line (HuH-7). Expression of HBV-specific major transcripts (3.5 and 2.1 kb), as well as HBV antigens (hepatitis B surface antigen and hepatitis B e and core antigens), was reduced about two- to fourfold by the presence of the HCV structural genes. In addition, the secretion of HBV viral particles, including the viral nucleocapsid and mature virion, was drastically suppressed about 20-fold. Analysis of the intracellular HBV core protein-associated nucleic acid indicated that the encapsidated HBV pregenomic RNA was similarly reduced about 14-fold. Deletion analysis of the HCV structural genes demonstrated that the core gene alone or the fragment containing the core protein's N-terminal 122 amino acid residues conferred the same level of suppressive activity as the full-length structural genes. By indirect immunofluorescence, we found that the core protein of HCV was located in the cytoplasm of transfected HuH-7 cells at day 3 posttransfection and was targeted to the nucleus at day 6. Thus, the kinetics of the suppressive effect exerted by HCV constructs matched the timing of core protein entrance into the nucleus. Our results substantiate the clinical finding that HBV markers are suppressed by superinfection with HCV and further imply that this inhibitory effect may occur in the processes of transcription and encapsidation of HBV pregenomic RNA and may be mediated by the core protein of HCV. The deduced amino acid sequence of the HCV core protein has revealed that it is a basic protein which contains a putative DNA-binding motif (SPRG), as well as triplicate nuclear localization signals and several putative protein kinase A and C recognition sites. These characteristics imply that the HCV core protein can also function as a gene-regulatory protein.

Protein species of the parvovirus minute virus of mice strain MVMp: involvement of phosphorylated VP-2 subtypes in viral morphogenesis

The pattern of induced protein species of the prototype strain of the parvovirus minute virus of mice was determined in permissive A9 mouse fibroblast cells by high-resolution two-dimensional gel electrophoresis. Identities of the viral proteins in the gels were assigned by probing two-dimensional blots with antisera raised against either purified capsids (recognizing VP-1 and VP-2) or specific coding regions of the nonstructural proteins (NS-1 and NS-2) expressed as beta-galactosidase fusion products in bacteria. All viral proteins showed posttranslational modifications, phosphate being a common substituent. The NS-1 protein migrated as a basic polypeptide in the pI range of 7.4 to 7.8 with multiple stages of modification and as a likely minor but hyperphosphorylated component in the neutral region of the gel. The NS-2 isoforms were resolved at a pI value close to 5.5 as three groups of unevenly phosphorylated polypeptides, each composed of at least two protein species. Both VP-1 and VP-2 structural polypeptides were induced as heterogeneous phosphoproteins. The major VP-2 protein could be resolved in the form of a consistent pattern of three abundant (a to c), two intermediate (d and e), and one meager (f) neutral isoelectric focusing species or subtypes. This posttranslational modification precedes and is uncoupled from viral assembly, and all of the VP-2 subtypes are packaged into empty capsids at the induced stoichiometry. However, intracellular full virions harbored additional phosphorylated subtypes (g to l) and a subtle rearrangement in the whole VP-2 composition, while mature virions purified from lysed cultures lacked these subtypes, coordinately with the emergence of six neutral VP-3 subtypes. Thus, the virion coat undergoes a chemical transition entailed by genome encapsidation, in which phosphates seem to play a major role, triggering the preferential proteolytic cleavage of the more acidic VP-2 subtypes to VP-3. Parvoviruses, with small coding capacity, may regulate some morphogenetic steps, such as assembly, genome encapsidation, and maturation, by posttranslational modifications of their structural proteins.