Differential binding of arfaptin 2/POR1 to ADP-ribosylation factors and Rac1

We first identified arfaptin as a protein that bound to GTP-ARFs (especiall y ARF1). However, a second group reported that PORI, a truncated form of ar faptin, bound to GTP-Rac1. Therefore, we examined the possibility that arfa ptin 2/POR1 was a common downstream effector for both ARF1 and Rac1. In thi s study, we found that constitutively active Rac1 or GTP-Rac1 showed neglig ible or no binding to arfaptin 2/POR1 in a yeast two-hybrid assay or a GST pull-down assay. However, wild-type or dominant negative Rac1 or Rac1 ligan ded to GDP showed strong binding. In contrast, constitutively active ARFs1, 5, and 6 showed binding, whereas the wild-type and dominant negative forms did not. Furthermore, the GTP-liganded ARFs bound arfaptin 2, whereas the GDP-bound forms showed little or no binding. Based on these observations, w e suggest that arfaptin 2/POR1 is a target protein for GTP-ARFs and for GDP -Rac1, and that it may be involved in interactions between the Rac and ARF signaling pathways. (C) 2001 Academic Press.