Human cytomegalovirus UL144 open reading frame: sequence hypervariability in low-passage clinical isolates

Human cytomegalovirus (HCMV) infects a number of organs and cell types in vivo, leading to the hypothesis that HCMV disease and tissue tropism may be related to specific sequence variants. A potential component of HCMV variant strains is the UL144 open reading frame (ORF), which encodes a homologue of the herpesvirus entry mediator, HveA, a member of the tumor necrosis factor receptor superfamily. Sequence analysis of the UL144 ORF in 45 low-passage clinical isolates demonstrated significant strain-specific variability. In individual isolates, nucleotide substitutions occur at up to 21% of the 531 positions, resulting in approximately the same percentage of substitutions in the predicted 176-amino-acid sequence. Phylogenetic analysis indicated that the nucleotide and amino acid sequences diverge into three major groups. For genotypic comparison, the known hypervariable region encompassing the proteolytic cleavage site of the glycoprotein B (gB) gene was also sequenced. All of the isolates could be typed according to the four known gB groups; however, the gB and UL144 sequence groups appeared to be phylogenetically unlinked. The predicted UL144 product homology with tumor necrosis factor receptor family members, along with the unexpectedly high level of sequence variability of the UL144 ORF, suggests that the predicted product may play a role in HCMV infectivity and subsequent host disease.

Calcium is required in reassembly of bovine papillomavirus in vitro

Papillomaviruses are small DNA viruses which infect and induce benign warts and sometimes malignant tumours in the epithelium of the skin or mucosa. The viruses do not replicate in conventional tissue culture systems and little is known about the requirements for virus assembly. We investigated the effect of ethylene glycol-bis(aminoethyl ether)-tetraacetic acid (EGTA) and dithiothreitol (DTT) treatment on the stability of bovine papillomavirus type 1 (BPV-1) particles in vitro. Removal of calcium ions by 11 mM EGTA at pH 8.0 together with reduction of disulfide bonds by 15 mM DTT destabilized BPV particles. Electron microscopy examination of treated particles showed that the BPV particles had been disrupted to capsomeres. Addition of exogenous calcium ions to the disruption buffer prevented virus destabilization. Adding calcium to the disrupted BPV particles resulted in the reassembly of disrupted particles. The reassembled particles were morphologically similar to intact BPV virions. We further quantified the efficiency of reassembly by focus formation assay. We recorded 500-fold less infectivity for reassembled BPV and 4-fold less haemagglutination activity compared to untreated BPV, pointing towards a decrease in the amount of reassembled particles recovered.

Carboxyl terminus of bovine papillomavirus type-1 L1 protein is not required for capsid formation

The papillomavirus major capsid protein L1 can assemble into capsids in vitro. To identify areas within the bovine papillomavirus type-1 L1 (BPV L1) protein that are important for virus assembly, we constructed a set of 24 baculovirus recombinants expressing BPV L1 deletion mutants that span the entire L1 open reading frame. Virus-like particle (VLP) formation of the L1 mutants was examined by electron microscopy. Wild-type (wt) BPV L1 expressed in recombinant baculovirus formed VLPs, while capsomeres and aggregates were seen for most of the mutants screened. However, the C-terminal truncation mutant, lacking the last 24 amino acids (delta C2), was observed to form VLPs (threefold more efficiently than wt BPV L1). This suggests that this C-terminal region of L1 protein is not critical for capsid formation. As capsids assembled from BPV L1 are able to agglutinate mouse red blood cells (RBC) by binding to a membrane protein, we tested the ability of the mutants to hemagglutinate mouse RBCs. Most aberrant capsids or aggregates derived from deletion mutants were unable to agglutinate the RBCs with the exception of deletion mutants delta 11 (aa 231-271), delta 14 (aa 291-331), delta 21 (aa 431-471), and the carboxyl-terminus truncation mutant delta C2.