The GTPase-containing subunits of heterotrimeric gtp-binding proteins. When dissociated from the heterotrimeric complex these subunits interact with a variety of second messenger systems. hydrolysis of GTP by the inherent GTPase activity of the subunit causes it to revert to its inactive (heterotrimeric) form. The gtp-binding protein alpha subunits are grouped into families according to the type of action they have on second messenger systems.