Identification and characterization of a novel Tre-2/Bub2/Cdc16 (TBC) protein that possesses Rab3A-GAP activity

The Tre-2/Bub2/Cdc16 (TBC) domain is a conserved protein motif that consists of approximately 200 amino acids and is thought to function as a specific Rab-GAP domain. Although more than 40 distinct TBC domain-containing proteins have been identified in humans, the GAP activity and specificity of most TBC proteins have never been determined. In this study we developed a novel method of screening for Rab3A-GAP and identified two TBC proteins (FLJ13130 and RN-tre) whose expression in PC12 cells was associated with exclusion of endogenous Rab3A molecules from dense-core vesicles. As expression of RN-tre caused fragmentation of the Golgi, which presumably resulted in the loss of dense-core vesicles themselves, we further characterized FLJ13130 as a candidate Rab3A-GAP. The results showed that expression of FLJ13130, but not of its catalytically inactive R134K mutant, greatly reduced the amount of GTP-Rab3A in living cells and promoted the GTPase activity of Rab3A in vitro. Unexpectedly, however, FLJ13130 also promoted the GTPase activity of Rab22A, Rab27A, and Rab35, but not of Rab2A or Rab6A. Based on these results, we propose that FLJ13130 is a novel type of Rab-GAP that exhibits broad GAP specificity and inactivates several distinct Rab isoforms, including Rab3A, just near the plasma membrane.