DIFFERENTIAL-MODES OF NUCLEAR-LOCALIZATION SIGNAL (NLS) RECOGNITION BY 3 DISTINCT CLASSES OF NLS RECEPTORS

The targeting of karyophilic proteins to nuclear pores is mediated via the formation of a nuclear pore-targeting complex, through the intera ction of nuclear localization signal (NLS) with its NLS receptor. Rece ntly, a novel human protein, Qip1, was identified from a yeast two-hyb rid system with DNA helicase Q1. This study demonstrates that Qip1 is a novel third class of NLS receptor that efficiently recognizes the NL S of the helicase Q1. Moreover, the data obtained in this study show t hat the specific interaction between Qip1 and the NLS of the helicase Q1 requires its upstream sequence of the minimal essential NLS. By usi ng purified recombinant proteins alone in the digitonin-permeabilized cell-free transport system, it was demonstrated that the two known hum an NLS receptors, Rch1 and NPI-1, are able to transport all the tested NLS substrates into the nucleus, while Qip1 most efficiently transpor ts the helicase Q1-NLS substrates, which contain its upstream sequence in so far as we have examined the system. Furthermore, in HeLa cell c rude cytosol, it was found that endogenous Rch1 binds to all the teste d NLS substrates, while the binding of endogenous NPI-1 is restricted to only some NLSs, despite the fact that NPI-1 itself shows binding ac tivity to a variety of NLSs. These results indicate that at least thre e structurally and functionally distinct NLS receptors exist in the hu man single cell population, and suggest that the nuclear import of kar yophilic proteins may be controlled in a complex manner at the NLS rec ognition step by the existence of a variety of NLS receptors with vari ous specificities to each NLS.