The Srp40 protein plays a dose-sensitive role in preribosome assembly or transport and depends on its carboxy-terminal domain for proper localization to the yeast nucleoskeleton

The yeast SRP40 gene product (Srp40p) is a highly serine-rich protein organized in three distinct domains. The roles of these domains in localizing Srp40p were determined. By indirect immunofluorescence microscopy, Srp40p localizes to punctate, sometimes fibrillar, subnuclear structures that might include the nucleolus. Its amino-terminal and medial domains are similar. They each start with a short basic stretch containing a nuclear localization signal, followed by a long acidic stretch with 76% serines; such acidic stretches are thought to mediate binding to ribosomal proteins in the nucleolus. Either domain is sufficient to determine nuclear localization of Srp40p. The Srp40p carboxy-terminal domain shows significant homology to the cognate domain of Nopp140, a mammalian nucleolar phosphoprotein of 140 kD. The carboxy-terminal domain alone, or fused to a reporter protein, displays a punctate localization outside the nucleus. Srp40p and Nopp140 share a highly homologous 39-residue motif within their similar carboxy-terminal domains. Inside or outside the nucleus, this motif is important to prevent Srp40p diffusion or degradation. These observations suggest that the punctate immunoreactive structure is nucleoskeletal and might result from Srp40p self-assembly. SRP40 genetically interacts with four mutants affected in stable RNA synthesis and one mutant blocked in protein translocation to the endoplasmic reticulum. Growth defects, but no translocation or rRNA transcription/maturation phenotypes, were observed upon SRP40 inactivation or strong overexpression. Together, these data point to a dispensable, dosage-sensitive, role of Srp40p in preribosome assembly or transport.