The Borna disease virus (BDV) p24 phosphoprotein is an abundant protein in
BDV-infected cultured cells and animal brains. Therefore, there is a possib
ility that binding of the p24 protein to cellular factor(s) induces functio
nal alterations of infected neural cells in the brain. To identify a cellul
ar protein(s) that interacts with BDV p24 protein, we performed far-Western
blotting with extracts from various cell lines. Using recombinant p24 prot
ein as a probe, we detected a 30-kDa protein in all cell lines examined. Bi
nding between the 30-kDa and BDV p24 proteins was also demonstrated using B
DV p24 affinity and ion-exchange chromatography columns. Microsequence anal
ysis of the purified 30-kDa protein revealed that its N terminus showed com
plete homology with rat amphoterin protein, which is a neurite outgrowth fa
ctor abundant in the brain during development. Mammalian two-hybrid and imm
unoprecipitation analyses also confirmed that amphoterin is a specific targ
et for the p24 protein in vivo. Furthermore, we showed that infection by BD
V, as well as purified p24 protein in the medium, significantly decreased c
ell process outgrowth of cells grown on laminin, indicating the functional
inhibition of amphoterin by interaction with the p24 protein. Immunohistoch
emical analysis revealed decreased levels of amphoterin protein at the lead
ing edges of BDV-infected cells. Moreover, the expression of the receptor f
or advanced glycation end products, of which the extracellular moiety is a
receptor for amphoterin, was not significantly activated in BDV-infected ce
lls during the process of extension, suggesting that the secretion of ampho
terin from the cell surface is inhibited by the binding of the p24 protein.
These results suggested that BDV infection may cause direct damage in the
developing brain by inhibiting the function of amphoterin due to binding by
the p24 phosphoprotein.