Molecular cloning of cDNA for BRab from the brain of Bombyx mori and biochemical properties of BRab expressed in Escherichia coli

From a brain cDNA library of Bombyx mori, we cloned cDNA for BRab, which encoded a 202-amino-acid polypeptide sharing 60-80% similarity with rab1 family members, To characterize its biochemical properties, cDNA for BRab was inserted into an expression vector (pGEX2T) and expressed in Escherichia coli as a glutathione S-transferase (GST) fusion protein. The recombinant protein was purified to homogeneity with glutathione S-Sepharose. The purified GST-BRab bound [S-35]-GTP gamma S and [H-3]-GDP with association constants of 1.5 x 10(6) M-1 and 0.58 x 106 M-1, respectively. The binding of [S-35]-GTP gamma S was inhibited with GTP and GDP, but with no other nucleotides, The GTP-hydrolysis activity was evaluated to be 5m mole/min/mole of BRab. In the presence of 6 mM MgCl2, bound [S-35]-GTPyS and [H-3]-GDP were exchanged with GTP gamma S most efficiently. These results suggest that BRab, having a higher affinity for GTP than GDP, converts from the GTP-bound state into the GDP-bound state by intrinsic GTP hydrolysis activity and returns to the GTP-bound state with the exchange of GDP with GTP.