While tubulin and related structural proteins also bind and hydrolyze GTP as part of their function to form intracellular tubules, these proteins utilize a distinct tubulin domain that is unrelated to the G domain used by signaling GTPases.
There are also GTP-hydrolyzing proteins that use a P-loop from a superclass other than the G-domain-containg one. Examples include the NACHT proteins of its own superclass and McrB protein of the AAA+ superclass.