Binding of baculoviruses to insect cells and fusion of the virus envelope to cell membranes are early events suggested to be affected by baculovirus enhancins. The binding of Autographa californica nucleopolyhedrovirus (AcMNPV) to the Spodoptera frugiperda cell line Sf21 and the fusion of the virus envelope to cell membranes were characterized. Virus binding assays demonstrated that AcMNPV budded virus (BV) bound to specific binding sites on Sf21 cells with an avidity of 2.3 x 10(10) M(-1). The cells displayed 3.1 x 10(3) specific binding sites per cell in a confluent monolayer. In addition, the effects of pH, buffer composition and cation concentration on the binding were examined. Using a fluorescent probe (R18) and fluorescence microscopy, the fusion of AcMNPV BV envelope to the cell membrane was directly visualized in living cells. It has been reported that Trichoplusia ni nucleopolyhedrovirus enters Sf21 cells by membrane fusion at the cell surface; however, the present studies confirmed the well established concept that adsorptive endocytosis is the major entry pathway for baculovirus BV infection. Membrane fusion kinetics and fluorescence microscopy demonstrated and verified that the envelope-cell membrane fusion was triggered by acidification. The effect of a T. ni granulovirus enhancin on virus binding and membrane fusion was examined, and no increase in activity was observed.


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