Repetitive segmental structure of the transducin beta subunit: homology with the CDC4 gene and identification of related mRNAs.

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Abstract
Retinal transducin, a guanine nucleotide regulatory protein (referred to as a G protein) that activates a cGMP phosphodiesterase in photoreceptor cells, is comprised of three subunits. We have identified and analyzed cDNA clones of the bovine transducin .beta. subunit that may be highly conserved or identical to that in other G proteins. From the cDNA nucleotide sequence of the entire coding region, the primary structure of a 340-amino acid protein was deduced. The encoded .beta. subunit has a Mr of 37,375 and is comprised of repetitive homologous segments arranged in tandem. Furthermore, significant homology in primary structure and segmental sequence exists between the .beta.-subunit and the yeast CDC4 gene product. The Mr 37,375 .beta. subunit polypeptide is encoded by a 2.9-kilobase (kb) mRNA. However, there exists in retina other .beta.-related mRNAs that are divergent from the 2.9-kb mRNA on the basis of oligonucleotide and primer-extended probe hybridizations. All mammalian tissues and clonal cell lines that have been examined contain at least two .beta.-related mRNAs, usually 1.8 and 2.9 kb in length. These results suggest that the mRNAs are the processed products of a small number of closely related genes or of a single highly complex .beta. gene.