Nuclear Transport of SRP Components

Nuclear import of SRP proteins: In eukaryotic cells, SRP mediates translocation of nascent polypeptides from the ribosome into the endoplasmic reticulum. Since SRP functions in the cytoplasm but contains an RNA molecule, a mechanism must exist for SRP RNA to be exported from the nucleus. We were the first to provide evidence that SRP RNA partly assembles with SRP proteins in the nucleus as a prerequisite for export to the cytoplasm. Recently, genetic analysis in yeast and immunolocalization studies with mammalian cells has confirmed that most SRP proteins are located to some extent in the nucleus and nucleolus (see Figure 1). Further, we found that human SRP19 can be imported into the nucleus by either of two nuclear transport factors, transportin and importin 8. Transportin is a well characterized member of the importin β family of nuclear transport factors;, importin 8 was only suspected to be an nuclear import factor until we confirmed its function (Figure 2).
immunolocalization of srp19
Figure 1. SRP19 colocalizes with fibrillarin, a nucleolar protein. Left: HeLa cells stained with antibody to fibrillarin. Middle: Cells stained with SRP19 antibody. Right: Merged image. Modified from Dean, K. et al., J. Cell Sci. 114: 3479, 2001. © The Company of Biologists, Ltd.
SRP19 in vitro nuclear import Figure 2. In vitro nuclear import of SRP19. Recombinant human SRP19 was purified, labeled with a fluorophore, mixed with purified nuclear import receptors, and incubated with nuclei of permeabilized HeLa cells in vitro to determine which receptor(s) would bring about nuclear accumulation of the SRP19 protein. The figure shows fluorescence micrographs of the results. In panel a, no import receptor was used. In the remaining panels, the receptors are: importin α/importin β heterodimer (panel b), importin β alone (panel c), transportin (panel d), importin 5 (panel e), importin 7 (panel f) and importin 8 (panel g). SRP19 import was most efficient with importin 8; however, transportin also mediated nuclear import of SRP19. Modified from Dean, K. et al., J. Cell Sci. 114: 3479, 2001. © The Company of Biologists, Ltd.

Current questions:
It is not known how any other SRP protein is imported into the nucleus, nor is it known how SRP19 interacts with transportin and importin 8. These two transport factors are different in structure and it remains to be determined whether they bind the same or different recognition sites in SRP19.
Back to Research Projects