Transportin is a major nuclear import receptor for c-Fos - A novel mode of cargo interaction

c-Fos, a component of the transcription factor AP-1, is rapidly imported into the nucleus after translation. We established an in vitro system using digitonin-permeabilized cells to analyze nuclear import of c-Fos in detail. Two import receptors of the importin beta superfamily, importin beta itself and transportin, promote import of c-Fos in vitro. Under conditions where importin beta-dependent transport was blocked, c-Fos still accumulated in the nucleus in the presence of cytosol. Inhibition of the transportin-dependent pathway, in contrast, abolished import of c-Fos. Furthermore, c-Fos mutants that interact with transportin but not with importin beta were efficiently imported in the presence of cytosol. Hence, transportin appears to be the predominant import receptor for c-Fos. A detailed biochemical characterization revealed that the interaction of transportin with c-Fos is distinct from the interaction with its established import cargoes, the M9 sequence of heterogeneous nuclear ribonucleoprotein A1 or the nuclear localization sequence of some basic proteins. Likewise, the binding sites on importin beta for its classic import cargo and for c-Fos can be separated. In summary, c-Fos employs a novel mode of receptor-cargo interaction. Hence, transportin may be as versatile as importin beta in recognizing different nuclear import cargoes.