Sorting of invertase signal peptide mutants in yeast dependent and independent on the signal‐recognition particle

There is growing evidence that yeast contains two efficient pathways of protein translocation across the endoplasmic reticulum membrane, one dependent on the signal‐recognition particle (SRP) and the other independent. Their specificity, however, is largely obscure. For higher eukaryotes it has been shown that a high average hydrophobicity of the core region with a minimal length around six or seven amino acids, as well as a stabilized α‐helix, are decisive structural features for translocation. Using yeast invertase as a secretory model protein, we have found that mutated signal sequences with Pro or Gly in the core, or having only four hydrophobic amino acids, are not functional in translocation across microsomal membranes of dog pancreas because they do not interact with the SRP. Expression of these mutant variants in Saccharomyces cerevisiae revealed that they are sorted independently of the SRP since translocation was not impaired in an SRP‐deficient yeast strain. In contrast to this, wild‐type invertase is translocated SRP‐dependently in wild‐type cells and shows a decreased translocation in SRP‐deficient cells. By overexpression of Srp54p, but not of Hsc70p, the translocation defect of wild‐type invertase in an SRP54 disruptant is restored. The data indicate that targeting of proteins to the endoplasmic reticulum in Saccharomyces cerevisiae seems to be more flexible than in higher eukaryotes as far as the structural requirements of signal sequences are concerned, and that the route taken is specified by the sequence.