Guanine nucleotide-binding proteins (G proteins) are involved as modulators or transducers in various transmembrane signaling systems. G proteins are composed of 3 units; alpha, beta and gamma. The alpha chain contains the guanine nucleotide binding site. In the N-terminal section; belongs to the G-alpha family. In the C-terminal section; belongs to the class-II aminoacyl-tRNA synthetase family. According to Ref.2, this sequence seems to be a hybrid that consists of parts of a G protein alpha subunit and a lysyl-tRNA synthetase.