-Spiders spin up to seven different types of silk and each type possesses different mechanical properties. The reports on base sequences of spider silk protein genes have gained importance as the mechanical properties of silk fibers have been revealed. This review aims to link recent molecular data, often translated into amino acid sequences and predicted three dimensional structural motifs, to known mechanical properties.

Infectious bursal disease virus (IBDV) encodes a 17-kDa nonstructural polypeptide known as VP5. This polypeptide is not essential for virus replication in vitro but it plays an important role in in vivo dissemination and pathogenesis. We have characterized the expression of VP5 in three eukaryotic systems: (i) IBDV-infected chicken embryo fibroblasts; (ii) BSC-1 cells infected with a recombinant vaccinia virus vector; and (iii) Cos-1 cells transiently transfected with a plasmid vector. Immunofluorescence analyses showed that upon expression VP5 accumulates within the plasma membrane. This finding was consistent with sequence-based topology predictions, indicating that VP5 is a class II membrane protein with a cytoplasmic N-terminus and an extracellular C-terminal domain. Brefeldin A treatment of VP5-expressing cells prevented the accumulation of this polypeptide in the plasma membrane, thus showing the requirement of an active exocytic pathway to reach that compartment. Expression of VP5 was shown to be highly cytotoxic. Induction of VP5 expression resulted in the alteration of cell morphology, the disruption of the plasma membrane, and a drastic reduction of cell viability. VP5-induced cytotoxicity was prevented by blocking its transport to the membrane with Brefeldin A. Our findings suggest that VP5 plays an important role in the release of the IBDV progeny.

A cDNA corresponding to the coding region of VP1, the putative RNA-dependent RNA polymerase, of infectious bursal disease virus (IBDV) was cloned and inserted into the genome of a vaccinia virus inducible expression vector. The molecular mass and antigenic reactivity of VP1 expressed in mammalian cells are identical to those of its counterpart expressed in IBDV-infected cells. The results presented here demonstrate that VP1 is efficiently incorporated into IBDV virus-like particles (VLPs) produced in mammalian cells coexpressing the IBDV polyprotein and VP1. Incorporation of VP1 into VLPs requires neither the presence of IBDV RNAs nor that of the nonstructural polypeptide VP5. Immunofluorescence, confocal laser scanning microscopy, and immunoprecipitation analyses conclusively showed that VP1 forms complexes with the structural polypeptide VP3. Formation of VP1-VP3 complexes is likely to be a key step for the morphogenesis of IBDV particles.

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Infectious bursal disease virus (IBDV) is the causative agent of an economically important poultry disease. Vaccinia virus recombinants expressing the IBDV mature structural capsid proteins VP2 and VP3 were generated by using vectors for inducible gene expression. Characterization of these recombinant viruses demonstrated that expression of VP2 leads to induction of apoptosis in a variety of mammalian cell lines. Transfection of cell cultures with a expression vector containing the VP2 coding region under the control of the immediate-early promoter-enhancer region of human cytomegalovirus also triggers programmed cell death. The apoptotic effect of VP2 is efficiently counteracted by coexpression of the proto-oncogene bcl-2. The results presented demonstrate that VP2 is a bona fide apoptotic inducer. Evaluation of the significance of this finding for the virus life cycle must await further research.