An acidic sequence of a putative yeast Golgi membrane protein binds COPII and facilitates ER export

We previously identified Sys1p as a high copy number suppressor of Ypt6 GTP ase-deficient yeast mutants that are defective in endosome-to-Golgi transpo rt. Here, we show that Sys1p is an integral membrane protein that resides o n a post-endoplasmic reticulum (ER) organelle(s). Affinity, studies with de tergent-solubilized yeast proteins showed that the C-terminal 53 amino acid tail of Sys1p binds effectively, to the cytoplasmic Sec23p-Sec24p COPII su bcomplex. This binding required a di-acidic Asp-Leu-Glu (DXE) motif, previo usly shown to mediate efficient ER export of the vesicular stomatitis virus glycoprotein in mammalian cells. In Sys1p. a Glu-Leu-Glu (EXE) sequence co uld not substitute for the (DXE) motif. Mutations of the (DXE) sequence res ulted in ER retention of similar to 30% of the protein at steady, state, wh ereas addition of the Sys1p tail to an ER-resident membrane protein led to an intracellular redistribution of the chimeric protein. Our study demonstr ates for the first time that, in yeast, a di-acidic sequence motif can act as a sorting signal for cargo selection during the formation of transport v esicles at the ER by direct binding to COPII component(s).