A PY-NLS nuclear targeting signal is required for nuclear localization and function of the Saccharomyces cerevisiae mRNA-binding protein Hrp1

Proteins destined for import into the nucleus contain nuclear localization signals (NLSs) that are recognized by import receptors termed karyopherins or importins. Until recently, the only nuclear import sequence that had been well defined and characterized was the classical NLS ( cNLS), which is recognized by importin alpha. However, Chook and coworkers ( Lee, B. J., Cansizoglu, A. E., Suel, K. E., Louis, T. H., Zhang, Z., and Chook, Y. M. ( 2006) Cell 126, 543 - 558) have provided new insight into nuclear targeting with their identification of a novel NLS, termed the PY-NLS, that is recognized by the human karyopherin beta 2/transportin (Kap beta 2) receptor. Here, we demonstrate that the PY-NLS is conserved in Saccharomyces cerevisiae and show for the first time that the PY-NLS is a functional nuclear targeting sequence in vivo. The apparent ortholog of Kap beta 2 in yeast, Kap104, has two known cargos, the mRNA-binding proteins Hrp1 and Nab2, which both contain putative PY-NLS- like sequences. We find that the PY-NLS- like sequence within Hrp1, which closely matches the PY-NLS consensus, is both necessary and sufficient for nuclear import and is also required for receptor binding and protein function. In contrast, the PY-NLS- like sequences in Nab2, which vary from the PY-NLS consensus, are not required for proper import or protein function, suggesting that Kap104 may interact with different cargos using multiple mechanisms. Dissection of the PY-NLS consensus reveals that the minimal PY-NLS in yeast consists of the C-terminal portion of the human consensus, R/H/KX2-5PY, with upstream basic or hydrophobic residues enhancing the targeting function. Finally, we apply this analysis to a bioinformatic search of the yeast proteome as a preliminary search for new potential Kap104 cargos.