Microinjected U snRNAs are imported to oocyte nuclei via the nuclear pore complex by three distinguishable targeting pathways.

The inhibitory effects of wheat germ agglutinin and mAb 414 on the nuclear import of all types of U snRNAs indicate that they cross the nuclear envelope through the nuclear pore complex. However, the import of different U snRNAs occurs by kinetically distinct targeting pathways that can be distinguished from one another by the competitive effects of free trimethylguanosine cap dinucleotide (m3GpppG) and P(Lys)-BSA, an efficient synthetic karyophile based on the nuclear localization signal of SV40 large T antigen. The import of U snRNAs that contain 5' m3GpppN caps and are complexed by Sm proteins (U1, U2, U4, and U5) is competed by coinjection with free m3GpppG, indicating a shared transport factor, but not by P(Lys)-BSA. The import of U6 snRNA, which lacks a m3GpppN cap and is not complexed by the Sm proteins, is competed by P(Lys)-BSA but not by free m3GpppG. Thus, by the criterion of kinetic competition, U6 snRNA import is identical to that of the karyophilic proteins P(Lys)-BSA and nucleoplasmin. Uniquely, the import of U3 snRNA, which contains a m3GpppN cap but does not bind Sm proteins is not competed by either free m3GpppG or P(Lys)-BSA. Thus, U3 snRNA appears to be imported by a novel third kinetic pathway.