A mutation in the secretion pathway of the yeast Yarrowia lipolytica that displays synthetic lethality in combination with a mutation affecting the signal recognition particle

In an attempt to identify proteins involved in the translocation step of pr otein secretion, a genetic screen was carried out in the yeast Yarrowia lip olytica. A conditional lethal mutant which has a defect in the 7S RNA of th e signal recognition particle was mutagenized and screened for second-site mutations that specifically exacerbate its temperature sensitivity. This ap proach had previously allowed the characterization of an endoplasmic reticu lum component, Sls1p, involved in protein translocation. A second mutation, sls2-1, was isolated that causes synthetic lethality when combined with th e 7S RNA mutation. On its own, the sls2-2 mutation confers a temperature-se nsitive growth phenotype. The secretory phenotype of the sls2 mutant consis ts in abnormal secretion of several polypeptides, and thus differs from the defect in secretory protein synthesis associated with the 7S RNA and sls2- 1 mutations. Two new Y. lipolytica genes were identified which can relieve the growth defect of sls2-1 cells. SLS2 itself and SSL2, a multicopy suppre ssor of the temperature sensitivity of the sls2 mutant. The SLS2 gene encod es a polypeptide that can potentially be farnesylated and phosphorylated, a nd shares some homology with an S. cerevisiae protein of unknown function. Ss12p resembles calmodulin-dependent serine/threonine protein kinases. Thes e two proteins may interact to regulate protein sorting.