Introduction to the Molecular Biology of Baculoviruses

Over the last 10 years, baculovirus expression vectors have become a very popular and effective means with which to produce recombinant proteins in large quantities (1 –5 ). Posttranslational modifications of the gene products of these insect viruses closely parallel glycosylation, fatty acid acylation, and phosphorylation in mammalian cells (reviewed in 6 ). Scaleup of insect cells in culture has also been largely perfected, making purification of large quantities of recombinant proteins a reality (7 ). In addition, baculoviruses offer an ecologically acceptable and effective alternative to chemicals for the control of forest and agricultural insect pests (8 ,9 ). Their demonstrated safety as expression vectors and pest management tools is the result of limited host specificity and lack of resemblance to mammalian viruses. The development of the baculovirus expression system was facilitated by the establishment of insect cell lines that support the replication of one subgroup, the nuclear polyhedrosis viruses (NPVs). The ability to propagate baculoviruses in cell culture has also allowed extensive study of their molecular biology (10 ). The model virus in these studies is the Autographa californica NPV (AcNPV). Although it was first isolated from the alfalfa looper (Autographa californica ), it multiplies readily in cell lines derived from both the fall armyworm (Spodopterafrugiperda ) and the cabbage looper (Trichoplusia ni ). Most expression vectors are based on AcNPV infection of Spodoptera frugiperda cells. However, the production of heterologous proteins in silkworm (Bombyx mori; Bm) larvae relies on infection with recombinant BmNPV (4 ). The baculovirus expression system is based on introduction of the foreign gene into nonessential regions of the viral genome through allelic replacement. Production of the recombinant protein is achieved following infection of insect cells or larvae with the newly engineered virus.