BSAP (Pax5)-importin α1 (Rch1) interaction identifies a nuclear localization sequence

BSAP (Pax5) is an essential transcription factor for early B cell and central nervous system development. In later B cell development, BSAP has been implicated in the regulation of 3' Ig enhancers and a number of B cell- specific genes. Previous studies have suggested a role for BSAP-interacting proteins in the regulation of the function of BSAP. Using the yeast two- hybrid system, we identified importin α1 (Rch1) as a BSAP-interacting protein. Importin α proteins have been shown to escort proteins into the nucleus through interaction with a nuclear localization signal (NLS), composed of short stretches of basic amino acids. A predicted NLS in BSAP (NKRKRDE, located at amino acids 195-201 in the central domain) was confirmed to be essential for interaction with importin α1 by the yeast two-hybrid assay. Physical interaction between BSAP and importin α1 was detected in vitro by a glutathione S-transferase (GST) pulldown assay. The NLS sequence in BSAP conferred nuclear localization to green fluorescent protein (GFP)- BSAP fusion proteins. Although the N-terminal paired (DNA-binding) domain of BSAP also conferred nuclear localization when coupled to green fluorescent protein, this domain did not bind to importin α1 in the yeast two-hybrid assay. The NLS sequence in the central domain of BSAP binds to the C-terminal 98-amino acid fragment of importin α1.