Distinct processes mediate glycoprotein and glycopeptide export from the endoplasmic reticulum in Saccharomyces cerevisiae.

Protein and peptide export from the Saccharomyces cerevisiae endoplasmic reticulum was examined in vitro using the secretory protein pro-alpha-factor and a synthetic tripeptide containing the acceptor site for N-linked glycosylation as substrates. The release of both glycosylated pro-alpha-factor and glycotripeptide from the endoplasmic reticulum was dependent on cytosol, temperature, and ATP. Antibodies against two proteins essential for the formation of transport vesicles, Sec23p and p105, inhibited glyco-pro-alpha-factor exit from the endoplasmic reticulum but did not affect the release of the glycosylated tripeptide. Furthermore, in contrast to pro-alpha-factor, the exported glycopeptide was not associated with a membrane fraction and did not acquire Golgi-specific alpha(1-6)-linked mannose residues. We conclude that the glycosylated tripeptide leaves the yeast endoplasmic reticulum by a route different from the secretory pathway, possibly through an ATP-driven pump.