Members of the ubiquitous GTPase superfamily regulate numerous cellular functions. A core group of eight GTPases are present in all domains of life: initiation factor 2, elongation factors Tu and G, protein secretion factors Ffh and FtsY, and the poorly characterized factors YihA, YchF, and HflX. While the first five members have well defined roles in the essential cellular process of protein synthesis, a role for YihA, YchF and HflX in this process has only recently been suggested. Here, a detailed kinetic analysis examining the interaction between HflX and its cellular partners is described. 50S and 70S ribosomal particles function as GTPase activating factors for HflX by stabilizing the nucleotide binding pocket of HflX, inducing a “GTPase activated” state. These data indicates a novel mode of GTPase activation, and suggests a role for HflX in regulating translation. / xii, 185 leaves : ill. (some col.) ; 28 cm