Are guanine nucleotide binding proteins a distinct class of regulatory proteins?

Proteins which bind guanine nucleotides are found in a diverse set of key regulatory positions. They are involved in hormone action, visual transduction, protein synthesis and microtubule assembly. In addition to their ability to bind guanine nucleotides these proteins possess several other common features. (i) They all have similar subunit composition, (ii) they can be ADP-ribosylated, (iii) their conformation changes depending on the nucleotide bound. These regulatory G-proteins have close functional homologies. Do they form a general class of regulatory proteins, like the protein kinases? Do they have a common evolutionary ancestry?