The coatomer-interacting protein Dsl1p is required for Golgi-to-endoplasmic reticulum retrieval in yeast

Sec22p is an endoplasmic reticulum (ER)-Golgi v-SNARE protein whose retriev al from the Golgi compartment to the endoplasmic reticulum (ER) is mediated by COPI vesicles. Whether Sec22p exhibits its primary role at the ER or th e Golgi apparatus is still a matter of debate. To determine the role of Sec 22p in intracellular transport more precisely, we performed a synthetic let hality screen. We isolated mutant yeast strains in which SEC22 gene functio n, which in a wild type strain background is non-essential for cell viabili ty, has become essential. In this way a novel temperature-sensitive mutant allele, dsl1-22, of the essential gene DSL1 was obtained. The dsl1-22 mutat ion causes severe defects in Golgi-to-ER retrieval of ER-resident SNARE pro teins and integral membrane proteins harboring a C-terminal KXXX retrieval motif, as well as of the soluble ER protein BiP/Kar2p, which utilizes the H DEL receptor, Erd2p, for its recycling to the ER. DSL1 interacts geneticall y with mutations that affect components of the Golgi-to-ER recycling machin ery, namely sec20-1, tip20-5, and COPI-encoding genes. Furthermore, we demo nstrate that Dsl1p is a peripheral membrane protein, which in vitro specifi cally binds to coatomer, the major component of the protein coat of COPI ve sicles.