In the yeast membrane traffic pathway, COPI mediated ER-Golgi retrograde traffic is essential for its indispensible role in recycling ER resident proteins, trafficking machinery molecules and maintaining the integrity of the Golgi apparatus. As the interdependence of ER-Golgi anterograde and retrograde pathways makes it difficult to separate them for individual study, some mechanisms and molecules responsible for the retrograde traffic remain elusive. Some trafficking components fulfilling key functions in the early secretory pathway are not essential, which indicates that multiples molecules or parallel pathways might be involved complementally. In present study, aiming to identify the potential retrograde traffic pathway, I conducted a genetic screen with a sec26 mutant. SEC2...[ Read more ]

In the yeast membrane traffic pathway, COPI mediated ER-Golgi retrograde traffic is essential for its indispensible role in recycling ER resident proteins, trafficking machinery molecules and maintaining the integrity of the Golgi apparatus. As the interdependence of ER-Golgi anterograde and retrograde pathways makes it difficult to separate them for individual study, some mechanisms and molecules responsible for the retrograde traffic remain elusive. Some trafficking components fulfilling key functions in the early secretory pathway are not essential, which indicates that multiples molecules or parallel pathways might be involved complementally. In present study, aiming to identify the potential retrograde traffic pathway, I conducted a genetic screen with a sec26 mutant. SEC26 is an essential gene encoding a subunit of COPI complex, the mutation of which results in the failure of sorting v-SNARE Sec22p in the COPI vesicles. However, the deficiency of Sec22p recycling doesn’t disturb the bi-directional traffic between ER and Golgi. In the screen, four genes were discovered that their missense mutations or null mutations were synthetic lethal with the sec26 mutant. TRS85, encoding a unique subunit of TRAPPIII complex, was among them. Besides trs85Δ, a trs85 allele with a conserved amino acid substitution was identified to be synthetic sick with the sec26 mutant. Previously, the TRAPPIII complex was considered to be only involved in initiating the organization of preautophagosome and tethering in autophagy pathway. Interestingly, I discovered that the trs85Δ, and the trs85 allele strains are defective in the anterograde delivering of Sec22p, and the Carboxypeptidase Y from ER to Golgi, which reveals a possible role of Trs85p in the early secretory pathway.

In conclusion, the TRAPP III complex, and its downstream effector GTPase Ypt1p might also function in the early secretory pathway, similarly with the TRAPPI, which has already been identified as tethering factor in the cis-Golgi membrane. The TRAPPIII complex might function in a parallel anterograde pathway that is essential to compensate the Sec22p when the later fails to be recycled, or the TRAPPIII-dependent pathway is required to deliver the traffic components that function in parallel with Sec22p in the retrograde direction.