Specific interaction of the yeast cis-Golgi syntaxin Sed5p and the coat protein complex II component Sec24p of endoplasmic reticulum-derived transport vesicles

The generation of transport vesicles at the endoplasmic reticulum (ER) depe nds on cytosolic proteins, which, in the form of subcomplexes (Sec23p/Sec24 p; Sec13p/ Sec31p) are recruited to the ER membrane by GTP-bound Sar1p and form the coat protein complex II (COPII). Using affinity chromatography and two-hybrid analyses, we found that the essential COPII component Sec24p, b ut not Sec23p, binds to the cis-Golgi syntaxin Sed5p, Sec24p/Sed5p interact ion in vitro was not dependent on the presence of [Sar1p.GTP]. The binding of Sec24p to Sed5p is specific; none of the other seven yeast syntaxins bou nd to this COPII component. Whereas the interaction site of Sec23p is withi n the N-terminal half of the 926-aa-long Sec24p (amino acid residues 56-549 ), Sed5p binds to the N- and C-terminal halves of the protein, Destruction by mutagenesis of a potential zinc finger within the N-terminal half of Sec 24p led to a nonfunctional protein that was still able to bind Sec23p and S ed5p. Sec24p/Sed5p binding might be relevant for cargo selection during tra nsport-vesicle formation and/or for vesicle targeting to the cis-Golgi.