Nuclear export of yeast signal recognition particle lacking Srp54p by the Xpo1p/Crm1p NES-dependant pathway

Background: The movement of macromolecules through the nuclear pores requir es energy and transport receptors that bind both cargo and nuclear pores. D ifferent molecules/complexes often require different transport receptors. T he signal recognition particle (SRP) is a conserved cytosolic ribonucleopro tein that targets proteins to the endoplasmic reticulum. Previous studies h ave shown that the export of SRP RNA from the nucleus requires trans-acting factors and that SRP may be at least partly assembled in the nucleus, but little else is known about how it is assembled and exported into the cytopl asm. Results: Of the six proteins that constitute the yeast SRP, we found that a il except Srp54p were imported into the nucleus. Four of these had nucleola r pools. The same four proteins are required for stability of the yeast SRP RNA scR1, suggesting that they assemble with the RNA in the nucleus to for m a central core SRP. This core SRP was a competent export substrate. Of th e remaining components, Sec65p entered the nucleus and was assembled onto t he core particle there, whereas Srp54p was solely cytoplasmic. The export o f SRP from the nucleus required the transport receptor Xpo1p/Crm1p and Yrb2 p, both components of the pathway that exports leucine-rich nuclear export signal (NES)-containing proteins from the nucleus. Conclusions: The SRP is assembled in the nucleus into a complex lacking onl y Srp54p. It is then exported through the NES pathway into the cytoplasm wh ere Srp54p binds to it. This transport route for a ribonucleoprotein comple x is so far unique in yeast.