Controlled conformational transitions in the MVM virion expose the VP1 N-terminus and viral genome without particle disassembly

Antisera were raised against peptides corresponding to the N-termini of cap sid proteins VP1 and VP2 from the parvovirus minute virus of mice. Epitopes in the 142-amino-acid VP1-specific region were not accessible in the great majority of newly released viral particles, and sera directed against them failed to neutralize virus directly or deplete stocks of infectious virion s. However, brief exposure to temperatures of 45 degrees C or more induced a conformational transition in a population of full virions, but not in emp ty viral particles, in which VP1-specific sequences became externally acces sible. In contrast, the VP2 N-terminus was antibody-accessible in all full, but not empty, particles without prior treatment. An electrophoretic mobil ity shift assay, in which particles were heat-treated and/or preincubated w ith antibodies prior to electrophoresis, confirmed this pattern of epitope accessibility, showing that the heat-induced conformational transition prod uces a retarded form of virion that can be supershifted by incubation with VP1-specific sera. The proportion of virions undergoing transition increase d with temperature, but at all temperatures up to 70 degrees C viral partic les retained structure-specific antigenic determinants and remained essenti ally intact, without shedding individual polypeptide species or subunits. H owever, despite the apparent integrity of its protective coat, the genome b ecame accessible to externally applied enzymes in an increasing proportion of virions through this temperature range, suggesting that the conformation al transitions that expose VP1 likely also allow access to the genome. Heat ing particles to 80 degrees C or above finally induced disassembly to polyp eptide monomers. (C) 1999 Academic Press.