Interaction of Bombyx mori nucleopolyhedrovirus BRO-A and host cell protein laminin

The Bombyx mori nucleopolyhedrovirus (BmNPV) contains five baculovirus repeated ORF ( bro) genes, all of which are expressed as delayed early genes. We have recently reported that BmNPV BRO proteins, specially BRO-A and BRO-C, contain a nucleic acid binding activity and are involved in nucleosome structures in nuclei of infected cells. To further understand the function of bro-a gene, we looked for factors interacting with BmNPV BRO-A using the yeast two-hybrid system. Fifteen clones obtained from a cDNA library of mock-infected cells and one from a library prepared at 2 h postinfection (p.i.) were found to comprise one distinct gene, which was identified as the Bombyx homolog (bLaminin) of Drosophila laminin beta1. A direct interaction between BRO-A and N-terminal region of bLaminin was demonstrated by in vitro pull-down experiments. Further pull-down assays using BmN cell extracts and anti-laminin antibodies also showed interaction of both proteins. In addition, two more clones were obtained from cDNA library of 12 h p.i. and were found to encode BRO-A itself, indicating that BRO-A forms an oligomer. Taken together, we propose that BRO-A may function as a laminin binding protein.