Evidence for a nuclear passage of nascent polypeptide-associated complex subunits in yeast

The nascent polypeptide-associated complex (NAC) has been found quantitatively associated with ribosomes in the cytosol by means of cell fractionation or fluorescence microscopy. There have been reports, however, that single NAC subunits may be involved in transcriptional regulation. We reasoned that the cytosolic location might only reflect a steady state equilibrium and therefore investigated the yeast NAC proteins for their ability to enter the nucleus. We found that single subunits of yeast NAC can indeed be transported into the nucleus and that this transport is an active process depending on different nuclear import factors. Translocation into the nucleus was only observed when binding to ribosomes was inhibited. We identified a domain of the ribosome-binding NAC subunit essential for nuclear import via the importin Kap123p/Pse1p-dependent import route. We hypothesize that newly translated NAC proteins travel into the nucleus to bind stoichiometrically to ribosomal subunits and then leave the nucleus together with these subunits to concentrate in the cytosol.