Diversity in the signals required for nuclear accumulation of U snRNPs and variety in the pathways of nuclear transport.

Abstract

Abstract. The requirements for nuclear targeting of a number of U snRNAs have been studied by analyzing the behavior of in vitro-generated transcripts after microinjection into the cytoplasm of Xenopus oocytes.Like the previously studied U1 snRNA, U2 snRNA is excluded from the nucleus when it does not have the 2,2,7mGpppN cap structure typical of the RNA polymerase II (pol II)-transcribed U snRNAs. Surprisingly, two other pol H-transcribed U snRNAs, U4 and U5, have a much less stringent requirement for the trimethyl cap structure. The 3,-monomethyl triphosphate cap structure of the RNA polymerase III-transcribed U6 snRNA, on the other hand, is shown not to play a role in nuclear targeting. Wheat germ agglutinin, which is known to prevent the import of many proteins into the nucleus, inhibits nuclear uptake of U6, but not of U1 or U5 snRNAs. Conversely, a 2,2,7mGpppG dinucleotide analogue of the trimethyl cap structure inhibits transport of the pol II U snRNAs, but does not detectably affect the transport of either U6 snRNA or a karyophilic protein. From these results it can be deduced that U6 enters the nucleus by a pathway similar or identical to that used by karyophilic proteins. The composite nuclear localization signals of the trimethyl cap-containing U snRNPs, however, do not function in the same way as previously defined nuclear targeting signals.

Distinct domains of the spinal muscular atrophy protein SMN are required for targeting to Cajal bodies in mammalian cells

“…Indeed, the association of the C-terminus of SMN with the zinc finger protein ZPR1 might regulate nuclear partitioning of SMN (Gangwani et al, 2005). If SMN can enter the nucleus only with the snRNPs, our results suggest the existence of different pathways for snRNP import, as previously postulated (Fischer et al, 1991).…”

“…Deletion of the SMN C-terminus, encoded by ex7, diminishes the oligomerization of SMN and its binding capacity for Sm proteins (Pellizzoni et al, 1999;Wang and Dreyfuss, 2001) and TGS1 (Mouaikel et al, 2003). Hypermethylation of the snRNA cap structure allows efficient nuclear migration of snRNPs (Mattaj, 1988;Fischer et al, 1991) and, in turn, their passage through CBs (reviewed in Cioce and Lamond, 2005). In light of the localization of some SMN mutants in CBs that are devoid of snRNPs, it seems that SMN retains snRNPs in CBs rather that snRNPs retain SMN proteins in CBs (Carvalho et al, 1999;Sleeman et al, 2001).…”

Characterization of the Nuclear Export Signal of Human T-Cell Lymphotropic Virus Type 1 Rex Reveals that Nuclear Export Is Mediated by Position-Variable Hydrophobic Interactions

“…To confirm that export was peptide specific and not a result of the fluorophore, we also tested conjugates that were reverse labeled. Hence, the injection of a mixture of FITC- It has previously been demonstrated that some types of transport through the nuclear pore can be inhibited by wheat germ agglutinin (WGA) (7), while other types are not affected (8). WGA can specifically block the export of mRNAs and small nuclear RNA (snRNA) through the nuclear pores of xenopus oocytes (5,22).…”

A structural, phylogenetic, and functional study of 15.5-kD/Snu13 protein binding on U3 small nucleolar RNA

“…For structural analysis, cold RNA transcripts were produced by transcription of phage M13mp9ϻT7-snR17A and its derivatives with the T7 RNA polymerase, using the conditions previously described (Mougin et al 1996), or by transcription of the hU3 PCR matrix with the SP6 RNA polymerase, using the conditions described by Fischer et al (1991 P] UTP (800 Ci/mmol) (ICN); and 40 U of T7 RNA polymerase (USB Pharmacia Biotech). Incubation and subsequent treatments of RNAs were done as previously described (Mougin et al 1996).…”