Canine parvovirus : endocytic entry and nuclear import

Abstract

Canine parvovirus (CPV), a nonenveloped DNA virus, emerged in 1978 as a new virus infecting dogs. The early phase of CPV life cycle involves a series of sequential events starting with the attachment of virion to receptors on the cell surface. Then, viruses are internalized by pH dependent endocytic pathway. Upon escape of the virus from endosomes into cytoplasm, the viral DNA and associated proteins are transported to the nucleus, where viral transcription and replication occurs. For assembly of new virions, viral proteins enter the nucleus. At present, relatively little is known about the nuclear targeting signals of parvoviral proteins. The present study was designed to investigate the detection of CPV and closely related parvoviruses with antibodies to synthetic peptides, to examine the mechanism by which CPV enters canine fibroma cells and to elucidate the mechanism of the nuclear transport of CPV proteins during the CPV infection. Our main findings were: first, it was evident that sequences derived from highly conserved VP2 and NS1 regions of CPV elicited antibodies which can be used in the detection of CPV and some related parvoviruses. Second, CPV entered a host cell via an endocytic route and microtubule-dependent delivery of CPV to late endosomes was required for productive infection. Low pH-treated CPV particles injected directly into the cytoplasm, thus avoiding the endocytic pathway, were unable to initiate progeny virus production, showing that factors of the endocytic route other than low pH were necessary for the initiation of infection by CPV. Third, the N-terminal region of the VP1 capsid protein contains a potential nuclear localization signal (NLS), which is sufficient alone to localize a carrier protein into the nucleus. A cluster of basic residues was important for localization activity.