Bovine parvovirus DNA-binding proteins: identification by a combined DNA hybridization and immunodetection assay

We have investigated the interaction between bovine parvovirus (BPV) capsid and non-capsid proteins and restriction fragments of the BPV genome by a combined DNA hybridization and immunodetection assay. 32P-labelled DNA was bound to nitrocellulose membranes bearing lysates of mock-infected and virus-infected cells whose proteins had been separated by SDS-polyacrylamide gel electrophoresis. The position of bound DNA was determined by autoradiography. The proteins on the membrane were still accessible to specific antibodies, allowing confirmation of the DNA-binding species by an immunodetection reaction. In 0.2 M-NaCl, BPV capsid proteins VP2 (72,000 daltons) and VP3 (62,000 daltons) bound the 0 to 16 map unit EcoRI fragment of BPV DNA which contained label in either the minus or plus strand. At higher salt concentration (0.5 M), only VP2 still bound DNA. Within this fragment, the capsid protein binding was restricted to those nucleotides between map units 0 and 4. No binding to capsid proteins was seen with the fragment spanning the middle of the genome and minor binding to VP3 was seen with the 5' end. Binding to the BPV non-capsid protein NP-1 was observed with the 0 to 16 map unit fragment when label was in the virion strand and to other possibly BPV-coded proteins when label was in the plus strand. The NP-1 binding was localized to map units 4 to 16. We did not detect binding to the BPV homologue(s) of the autonomous parvovirus non-capsid protein NS1, due in part to its low concentration in the cell lysates used. Points of the parvovirus replication cycle at which DNA-binding proteins may serve controlling functions are discussed.