In the secretory pathway, proteins are transported from the endoplasmic reticulum (ER) to Golgi in COPII-coated vesicles. The COPII protein complex is comprised of Sar1p, Sec23p, Sec24p, Sec13p and Sec31p and is essential for vesicle formation as well as cargo recruitment on ER membranes. The selective and efficient recruitment of proteins into COPII-coated vesicles is mainly achieved via signal-mediated or receptor-dependent sorting. In yeast cells, Sed5p (Qa-SNARE), Sec22p (R-SNARE), Bet1p (Qc-SNARE) and Bos1p (Qb-SNARE) form a SNARE complex that is essential for ER-Golgi transport and these SNAREs are known to be efficiently packaged into newly forming COPII-coated vesicles. Although Sec24p-binding motifs have been identified for Sed5p, Sec22p and Bet1p, via structural and in vitro...[ Read more ]

In the secretory pathway, proteins are transported from the endoplasmic reticulum (ER) to Golgi in COPII-coated vesicles. The COPII protein complex is comprised of Sar1p, Sec23p, Sec24p, Sec13p and Sec31p and is essential for vesicle formation as well as cargo recruitment on ER membranes. The selective and efficient recruitment of proteins into COPII-coated vesicles is mainly achieved via signal-mediated or receptor-dependent sorting. In yeast cells, Sed5p (Qa-SNARE), Sec22p (R-SNARE), Bet1p (Qc-SNARE) and Bos1p (Qb-SNARE) form a SNARE complex that is essential for ER-Golgi transport and these SNAREs are known to be efficiently packaged into newly forming COPII-coated vesicles. Although Sec24p-binding motifs have been identified for Sed5p, Sec22p and Bet1p, via structural and in vitro biochemical approaches, the physiological consequences of deficiencies in SNARE-Sec24p interactions have not been extensively explored. I have examined the physiological consequences of amino acid substitutions to SNAREs that knockdown their binding to Sec24p in vitro assays. I find that in the absence of a Sec24p-binding motif, Sed5p, Sec22p or Bet1p were inefficiently exported from the ER. Nonetheless, despite the profound transport delay of these SNAREs, cell growth and protein transport (in general) were not significantly affected in cells singly expressing these Sec24p-binding deficient SNAREs.

In addition, I find that amino acid substitutions to two di-basic motifs in Bet1p’s C-terminal cytoplasmic domain could knockdown its binding to Sar1p in an in vitro binding assay and release Sec24p-binding deficient bet1p from the ER in living cells, suggesting that the Bet1p-Sar1p interaction is involved in the ER export of Bet1p. In contrast, Bos1p appears not to directly bind to Sec24p, and might employ different mechanisms to exit the ER.