THE SACCHAROMYCES-CEREVISIAE SEC20 GENE ENCODES A MEMBRANE GLYCOPROTEIN WHICH IS SORTED BY THE HDEL RETRIEVAL-SYSTEM

The SEC20 gene product (Sec20p) is required for endoplasmic reticulum (ER) to Golgi transport in the yeast secretory pathway. We have cloned the SEC20 gene by complementation of the temperature sensitive phenotype of a sec20-1 strain. The DNA sequence predicts a 44 kDa protein with a single membrane-spanning region; Sec20p has an apparent molecular weight of 50 kDa and behaves as an integral membrane protein with carbohydrate modifications that appear to be O-linked. A striking feature of this protein is its C-terminal sequence, which consists of the tetrapeptide HDEL. This signal is known to be required for the retrieval of soluble ER proteins from early Golgi compartments, but has not previously been observed on a membrane protein. The HDEL sequence of Sec20p is not essential for viability but helps to maintain intracellular levels of the protein. Depletion of Sec20p from cells results in the accumulation of an extensive network of ER and clusters of small vesicles. We suggest a possible role for the SEC20 product in the targeting of transport vesicles to the Golgi apparatus.