Nucleic Acids Research, Vol 24, Issue 10 1829-1836, Copyright © 1996 by Oxford University Press
C Marshallsay, A Dickmanns, FR Bischoff, H Ponstingl, E Fanning and R Luhrmann
GTP-hydrolysis, the small ras-related GTP-binding protein Ran and its
cognate guanosine nucleotide exchange factor, the RCC1 gene product, have
recently been identified as essential components of the protein nuclear
import pathway. In this report we use three independent approaches to
investigate the role of these components in U1 snRNP nuclear import in
somatic cells. (i) Using a somatic cell based in vitro nuclear import
system we show that U1 snRNP nuclear import, in marked contrast to protein
transport, is not significantly inhibited by non-hydrolyzable GTP-analogs
and is therefore unlikely to require GTP- hydrolysis. (ii) Using the
dominant negative Ran mutant RanQ69L, which is defective in GTP-hydrolysis,
we show that Ran-mediated GTP- hydrolysis is not essential for the nuclear
import of U1 snRNP in microinjected cultured cells. (iii) Using a cell line
expressing a thermolabile RCC1 gene product, we show that the nuclear
accumulation of microinjected U1 snRNP is not significantly affected by
RCC1 depletion at the non-permissive temperature, indicating that RCC1
function is not essential for U-snRNP nuclear import. Based on these
observations we conclude that protein and U-snRNP nuclear import in somatic
cells differ in their requirements for GTP-hydrolysis, and Ran or RCC1
function. Based on these results, the substrates for nucleocytoplasmic
exchange across the NPC can be divided into two classes, those absolutely
requiring Ran, including protein import and mRNA export, and those for
which Ran is not essential, including U- snRNP nuclear import, together
with tRNA and U1 snRNA nuclear export.
ARTICLES
In vitro and in vivo evidence that protein and U1 snRNP nuclear import in somatic cells differ in their requirement for GTP-hydrolysis, Ran/TC4 and RCC1 [published erratum appears in Nucleic Acids Res 1996 Jul 1;24(13):2626]
Institute for Molecular Biology and Tumour Research, Philipps University, Marburg, Germany.
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