Biogenesis of the signal recognition particle (SRP) involves import of SRPproteins into the nucleolus, assembly with the SRP-RNA, and Xpo1p-mediatedexport

The signal recognition particle (SRP) targets nascent secretory proteins to the ER, but how and where the SRP assembles is largely unknown. Here we an alyze the biogenesis of yeast SRP, which consists of an RNA molecule (scR1) and six proteins, by localizing all its components. Although scR1 is cytop lasmic in wild-type cells, nuclear localization was observed in cells lacki ng any one of the four SRP "core proteins" Srp14p, Srp21p, Srp68p, or Srp72 p. Consistently, a major nucleolar pool was detected for these proteins. Se c65p, on the other hand, was found in both the nucleo-plasm and the nucleol us, whereas Srp54p was predominantly cytoplasmic. Import of the core protei ns into the nucleolus requires the ribosomal protein import receptors Pse1p and Kap123p/Yrb4p, which might, thus, constitute a nucleolar import pathwa y. Nuclear export of scR1 is mediated by the nuclear export signal receptor Xpo1p, is distinct from mRNA transport, and requires, as evidenced by the nucleolar accumulation of scR1 in a dis3/rrp44 exosome component mutant, an intact scR1 3' end. A subset of nucleoporins, including Nsp1p and Nup159p (Rat7p), are also necessary for efficient translocation of scR1 from the nu cleus to the cytoplasm. We propose that assembly of the SRP requires import of all SRP core proteins into the nucleolus, where they assemble into a pr e-SRP with scR1. This particle can then be targeted to the nuclear pores an d is subsequently exported to the cytoplasm in an Xpo1p-dependent way.