Characterization of the major nuclear localization signal of the Borna disease virus phosphoprotein

Borna disease virus (BDV) replicates and transcribes its negative-sense RNA genome in the nucleus. The BDV phosphoprotein (P) is localized in the nucl eus of infected cells and cells transfected with P expression constructs, T o identify the nuclear localization signal (NLS) of P, COS-7 cells were tra nsfected with wild-type or mutant forms of P fused with green fluorescent p rotein (GFP), Whereas GFP alone was exclusively cytoplasmic, P or P-GFP wer e nuclear. Analysis of carboxy- and aminoterminal truncation mutants of P i ndicated that amino acids (aa) 20-37 are sufficient to promote efficient nu clear accumulation of the fusion protein. Residual nuclear import of GFP wa s observed with portions of P including aa 33-134 or aa 134-201, suggesting the presence of additional NLS motifs, The major NLS of P appears to be bi partite, It consists of two basic aa domains, R22RER25 and R30PRKIPR36, sep arated by four non-basic aa, S26GSP29.