sentence: In stark contrast , no reaction was observed with the covalent aGDP-modified GTPases Rab1b – aGDP and Rab1b – aGDP , thus showing that no GEF-mediated displacement of the covalently bound nucleotide occurred , as expected.The interaction with GEFs is expected to be quite weak for GTPases with covalently attached nucleotides , since the basis of the mechanism of GEF action is that they interact weakly ( K values in the range of 10 – 100 μ M ) with nucleotide-bound states of the proteins , but very strongly ( K values in the nM to pM range ) with the nucleotide-free proteins17 .