Recent developments on virus-like particles have
demonstrated their potential in transfecting eucaryotic
cells. In the case of particles based on the major coat
protein VP1 of polyoma virus, transfection occurs via binding
of VP1 to sialic acids. Since sialic acid is present on
almost every eucaryotic cell line, this results in an unspecific
cell targeting. Generation of a cell-type specificity of
this system would imply the presentation of a new function
on the surface of VP1. To analyze whether a new functional
protein can be placed on VP1, we inserted dihydrofolate
reductase from Escherichia coli as a model protein.
The effect of such an insertion on both VP1 and the inserted
protein was investigated, respectively. The function of
VP1, like the formation of pentameric capsomers and its
ability to assemble into capsids, was not influenced by
the insertion. The inserted dihydrofolate reductase showed
major changes when compared to the wild-type form. The
thermal stability of the enzyme was dramatically reduced
in the fusion protein; nevertheless, the dihydrofolate
reductase proved to be a fully active enzyme with only
slightly increased KM values for its
substrates. This model system provides the basis for further
modifications of the VP1 protein to achieve an altered
surface of VP1 with new properties.