Distinct Forms of the β Subunit of GTP-Binding Regulatory Proteins Identified by Molecular Cloning

Two distinct β subunits of guanine nucleotide-binding regulatory proteins have been identified by cDNA cloning and are referred to as β1and β2subunits. The bovine transducin β subunit (β1) has been cloned previously. We have now isolated and analyzed cDNA clones that encode the β2subunit from bovine adrenal, bovine brain, and a human myeloid leukemia cell line, HL-60. The 340-residue Mr37,329 β2protein is 90% identical with β1in predicted amino acid sequence, and it is also organized as a series of repetitive homologous segments. The major mRNA that encodes the bovine β2subunit is 1.7 kilobases in length. It is expressed at lower levels than β1subunit mRNA in all tissues examined. The β1and β2messages are expressed in cloned human cell lines. Hybridization of cDNA probes to bovine DNA showed that β1and β2are encoded by separate genes. The amino acid sequences for the bovine and human β2subunit are identical, as are the amino acid sequences for the bovine and human β1subunit. This evolutionary conservation suggests that the two β subunits have different roles in the signal transduction process.