Fig. 9. Mechanism of the role of IQGAP1 in regulating secretion. (A) CDC42 negatively regulates IQGAP1 secretion and is abrogated by IQGAP1-N1 expression. Pancreatic β-cells stably co-expressing CDC42WT (depicted as 42) and a IQGAP1 domain (on the x-axis) were assayed for insulin exocytosis as described in the Materials and Methods. CDC42WT stable cells expressing the V5 vector were used for control (42-V). Means ± s.d. for n=6 are shown. Values were normalized to cells that incorporated the vector (42-V) as a control. (B) IQGAP1 increases cellular CDC42-GTP. Left: the expression level of the V5-IQGAP1 constructs the cells used for the pull-down. Middle: positive (GTPS) and negative (GDP) controls for the pull-down experiment. Right: GST construct expressing the CDC42-binding (CRIB) domain of PAK (PBD) was used to pull-down active CDC42 (GTP-CDC42) from β-cells expressing IQGAP1 (F1), IQGAP1-C (C2), IQGAP1-N (N1) or vector control (V). Bottom: western blot of 10% of the total protein used for IP demonstrating equal input of cellular CDC42. (C) A model for the mechanism of the role of IQGAP1 in secretion as a conformational switch. In protein synthesis/exocytosis, IQGAP1 operates in a closed form generated by folding of the C-terminus. External or internal signals lead to the phosphorylation of IQGAP1 at a C-terminus serine (pS) and to the binding and activation of CDC42, perhaps to switch effector molecules in order to regulate exocytosis (on or off) or influence alternate cellular functions, such as cell division.