A ROLE FOR RIBOSOMAL-PROTEIN L5 IN THE NUCLEAR IMPORT OF 5S RIBOSOMAL-RNA IN XENOPUS OOCYTES

Prior to ribosome assembly, 5S ribosomal RNA (5S rRNA) binds to riboso mal protein L5 to form a stable ribonucleoprotein particle (5S RNP). W e have analyzed the role of L5 binding in the nuclear targeting of 5S rRNA in Xenopus oocytes, and have compared the nuclear import pathway of 5S RNPs with other karyophilic molecules. Nuclear import of in vitr o-generated 5S RNPs was found to be sensitive to three general inhibit ors of nuclear pore complex-mediated translocation: ATP depletion, chi lling, and wheat germ agglutinin. The initial rate and extent of net n uclear import was threefold greater with preassembled 5S RNPs than wit h 5S rRNA microinjected alone, suggesting that L5 binding is a prerequ isite for nuclear accumulation. Nuclear import of 5S rRNA/5S RNPs is a facilitated process dependent on limiting factors, since nuclear impo rt exhibited saturation kinetics. Not only was nuclear import of label ed 5S rRNA reduced in the presence of excess unlabeled 5S rRNA, but al so in the presence of the synthetic karyophilic protein P(lys)-BSA. In contrast, import was not inhibited by U1 small nuclear RNA (snRNA) or U3 small nucleolar RNA (snoRNA). 5S rRNA/5S RNP nuclear import theref ore appears to follow a pathway of molecular interactions similar to m any karyophilic proteins, but not the methylguanosine cap dependent U1 snRNA pathway or the cap-independent U3 snoRNA pathway. (C) 1996 Acad emic Press, Inc.