Molecular cloning and characterization of a cDNA for the alpha subunit of a G protein from rice

We report the isolation of a cDNA for the alpha subunit of a G protein from rice (Oryza sativa L. cv. Nipponbare). The cDNA contained an open reading frame that encoded a protein of 380 amino acid residues with a mol wt of 44,204. We designated this polypeptide RGA1 (rice G protein alpha subunit 1). The amino acid sequence of RGA1 was 77% and 86% identical to the sequences of alpha subunits from Arabidopsis thaliana and tomato (products of GPA1 and TGA1), respectively, and 42% to 69% identical to sequences of mammalian alpha subunits. The regions essential for binding to GTP were preserved throughout all alpha subunits from higher plants and mammals. However, the C-terminal amino acid sequence, which has been proposed to be a receptor-binding region, of RGA1 was different not only from the analogous sequences of mammalian alpha subunits but also from those of the products of GPA1 and TGA1. The mRNA for RGA1, of 1.7 kb in length, was found in the roots and in the etiolated and greening leaves of rice, suggesting that RGA1 might be a protein that is expressed constitutively.