G-proteins are some of the most important and abundant enzymes, yet their intrinsic nucleotide hydrolysis reaction is notoriously slow and must be accelerated in vivo. Recent experiments on dynamin and GTPases involved in ribosome assembly have demonstrated that their hydrolysis activities are stimulated by potassium ions. This article presents the hypothesis that cation-mediated activation of G-proteins is more common than currently realised, and that such GTPases represent a structurally and functionally unique class of G-proteins. Based on sequence analysis we provide a list of predicted cation-dependent GTPases, which encompasses almost all members of the TEES, Obg-HflX, YqeH-like and dynamin superfamilies. The results from this analysis effectively re-define the conditions under which many of these G-proteins should be studied in vitro.