Regulation of the RalGAP Complex by the 14-3-3 Protein in Adipocytes
Type 2 diabetes is caused in part by the body's development of insulin resistance in the liver", muscle, and fat cells, where glucose is absorbed into the cells after insulin stimulation. RalA is a G protein in the insulin signaling pathway that initiates the translocation of glucose transporter proteins to the plasma membrane. Previous studies indicate that Akt phosphorylation and the RalGAP Complex (RGC), a GTPase activating protein (GAP) complex consisting of two proteins (RGCl and RGC2), are involved in activating RaIA. Proteins similar to the RGC have been shown to be negatively regulated by 14-3-3 proteins binding to phosphoserines or phosphofhreonines. The 14-3-3 regulatory protein may also inhibit 'RGC activity. We investigated the hypothesis that 14- 3-3 binds to the RGC after insulin stimulation on specific phosphorylated sites. Adipocytes were stimulated with insulin to test for insulin-dependent 14-3-3 binding and with calf intestinal phosphatase to test for phosphate-dependent 14-3-3 binding. Protein pull-down assays were performed with GST 14-3-3. agarose beads and each sample was immunoblotted for RGCl and RGC2. We found an increase in 14-3-3 and RGC interaction after insulin stimulation and an absence of protein interaction after the addition of phosphatase. RGC mutants were also made using site-dire~ted mutagenesis PCR in order to start the investigation of whether or not the eight Akt phosphorylation sites on the RGC overlap with 14-3-3 binding sites. We produced six successful mutants. These findings suggest that 14-3-3 is the likely regulator of the RGC. However more testing will have to be done to confirm this proposition and to find the specific binding site for 14-3-3. A better understanding of RGC regulation and the insulin signaling pathway as a whole could lead to improved treatments for type 2 diabetes.