Abstract : The Neurofibromatosis type 1 (NF1) gene encodes a large tumor suppressor protein, neurofibromin, which is a Ras GTPase-activating protein (RasGAP) activity. Although the NF1 gene was identified over a decade ago, the biological roles of neurofibromin in cellular processes remain unclear. Therefore it is crucial for therapy and developing new drugs for NF1 patients to elucidate how the RasGAP activity of neurofibromin is controlled. To achieve this goal, it is also important to identify regulatory elements for neurofibromin. We are investigating the molecular mechanisms by which the Ras GAP activity of the yeast neurofibromin homologs Ira1/2 is regulated as a model to understand human NF1. We have found that the kelch Gb subunit mimics Gpb1/2 interact with Ira1/2 and control the Ras GAP activity of Ira1/2. Here, we found that the Gpb1/2 proteins are localized to the cell membrane in a Gpa2 dependent manner and function at the cell membrane. Gpb1/2 bind to the C-terminus of Ira1/2 and stabilize the Ira1/2 proteins. Moreover we also identified a Gpb1/2 binding domain near the C-terminus of Ira1/2 (GBD) that is significantly conserved in neurofibromin homologs, including a human counterpart. Therefore, similar regulatory mechanisms might be conserved in evolution.