SUBUNITS OF THE SACCHAROMYCES-CEREVISIAE SIGNAL RECOGNITION PARTICLE REQUIRED FOR ITS FUNCTIONAL EXPRESSION

The signal recognition particle (SRP) is an evolutionarily conserved e d ribonucleoprotein (RNP) complex that functions in protein targeting to the endoplasmic reticulum (ER) membrane. Only two protein subunits of the SRP, Srp54p and Sec65p, and the RNA subunit, scR1, were previou sly known in the yeast Saccharomyces cerevisiae. Purification of yeast SRP by immunoaffinity chromatography revealed five additional protein s. Amino acid sequencing and cloning of the genes encoding four of the se proteins demonstrated that the yeast SRP contains homologs (termed Srp14p, Srp68p and Srp72p) of the SRP14, SRP68 and SRP72 subunits foun d in mammalian SRP. The yeast SRP also contains a 21 kDa protein (term ed Srp21p) that is not homologous to any protein in mammalian SRP. An additional 7 kDa protein mag correspond to the mammalian SRP9. Disrupt ion of any one of the four genes encoding the newly identified SRP pro teins results in slow cell growth and inefficient protein translocatio n across the ER membrane. These phenotypes are indistinguishable from those resulting from the disruption of genes encoding SRP components i dentified previously. These data indicate that a lack of any of the an alyzed SRP components results in loss of SRP function. ScR1 RNA and SR P proteins are at reduced levels in cells lacking any one of the newly identified proteins. In contrast, SRP components are present at near wild type levels and SRP subparticles are present in cells lacking eit her Srp54p or Sec65p. Thus Srp14p, Srp21p, Srp68p and Srp72p, but not Sec65p or Srp54p, are required for stable expression of the yeast SRP.