Distinct regulations of ARF1 by two Aplysia Sec7 isoforms.

Sec7 protein is a guanine nucleotide exchange factor in the ADP-ribosylation factor (ARF) family of small GTP-binding proteins. Aplysia Sec7 proteins (ApSec7s) play many roles in neurite outgrowth and synaptic facilitation in Aplysia neurons. However, the binding property of Aplysia ARF1 by ApSec7 isoforms has not been examined. In this study, we found that the cloned Aplysia ARF1 (ApARF1) protein only localized to the Golgi complex when it was expressed alone in HEK293T cells; however, if ApARF1 was co-expressed with plasma membrane-targeted ApSec7, it localized to both the plasma membrane and the Golgi complex via association with the Sec7 domain of ApSec7. Moreover, in HEK293T cells expressing both ApARF1 and another Sec7 isoform, ApSec7(VPKIS), the pleckstrin homology domain of ApSec7(VPKIS) associated with ApARF1, resulting in its localization to the Golgi complex. Overall, we propose a model in which ApSec7(VPKIS) activates ApARF1 in the Golgi complex, while ApSec7 recruits ApARF1 to the plasma membrane where it activates ApARF1/6 downstream signaling.