DELTA-COP AND ZETA-COP, 2 COATOMER SUBUNITS HOMOLOGOUS TO CLATHRIN-ASSOCIATED PROTEINS, ARE INVOLVED IN ER RETRIEVAL

Two new thermosensitive yeast mutants defective in retrieval of dilysi ne-tagged proteins from the Golgi back to the endoplasmic reticulum (E R) were characterized. While both ret2-1 and ret3-1 were defective for ER retrieval, only ret2-1 exhibited a defect in forward ER-to-Golgi t ransport at the non-permissive temperature. Coatomer (COPI) from both mutants could efficiently bind dilysine motifs in vitro. The correspon ding RET2 and RET3 genes were cloned by complementation and found to e ncode the delta and zeta subunits of coatomer respectively. Both prote ins show significant homology to clathrin adaptor subunits. These resu lts emphasize the role of coatomer in retrieval of dilysine-tagged pro teins back to the ER, and the similarity between clathrin and coatomer coats.