Dimerisation‐dependent GTPase reaction of MnmE: how potassium acts as GTPase‐activating element

MnmE, a Guanine nucleotide‐binding protein conserved between bacteria and man, is involved in the modification of tRNAs. Here we provide biochemical and X‐ray structural evidence for a new GTP‐hydrolysis mechanism, where the G‐domains of MnmE dimerise in a potassium‐dependent manner and induce GTP hydrolysis. The structure in the presence of GDP‐AlF x and potassium shows how juxtaposition of the subunits induces a conformational change around the nucleotide which reorients the catalytic machinery. A critical glutamate is positioned such as to stabilise or activate the attacking water. Potassium provides a positive charge into the catalytic site in a position analogous to the arginine finger in the Ras‐RasGAP system. Mutational studies show that potassium‐dependent dimerisation and GTP hydrolysis can be uncoupled and that interaction between the G‐domains is a prerequisite for subsequent phosphoryl transfer. We propose a model for the juxtaposition of G‐domains in the full‐length protein and how it induces conformational changes in the putative tRNA‐modification centre.