Rein et al. begin with the observation that Arf1p's GTPase-activating protein, ARF-GAP, recruits Arf1p to membranes. ARF-GAP must be added first to get subsequent binding of Arf1p and coatomer, but ARF-GAP does not have to be present during the binding of Arf1p. Rather, it seems that ARF-GAP induces a conformational change in the v-SNARE proteins (converting them to a protease-resistant form), thus allowing Arf1p and coatomer to bind.
This conformation-altering function is separable from the later GAP function of ARF-GAP, and it distinguishes the Golgi-to-ER system from the ER-to-Golgi system. In the latter case, v-SNAREs are taken up into vesicles via direct interactions with coat proteins, but there is no evidence for a conformational change. ▪