Characterization of an Unusual Importin α Binding Motif in the Borna Disease Virus p10 Protein That Directs Nuclear Import

Nuclear import of many cellular and viral proteins is mediated by short nuclear localization signals (NLS) that are recognized by intracellular receptor proteins belonging to the importin/karyopherin α and β families. The primary structure of NLS is not well defined, but most contain at least three basic amino acids and harbor the relative consensus sequence K(K/R) X(K/R). We have studied the nuclear import of the Borna disease virus p10 protein that lacks a canonical oligobasic NLS. It is shown that the p10 protein exhibits all characteristics of an actively transported molecule in digitonin-permeabilized cells. Import activity was found to reside in the 20 N-terminal p10 amino acids that are devoid of an NLS consensus motif. Unexpectedly, p10-dependent import was blocked by a peptide inhibitor of importin α-dependent nuclear translocation, and the transport activity of the p10 N-terminal domain was shown to correlate with the ability to bind to importin α. These findings suggest that nuclear import of the Borna disease virus p10 protein occurs through a nonconventional karyophilic signal and highlight that the cellular importin α NLS receptor proteins can recognize nuclear targeting signals that substantially deviate from the consensus sequence.