Nitcotiana tabacum cDNAs encoding α and β subunits of a heterotrimeric GTP-binding protein isolated from hairy root tissues

Heterotrimeric GTP-binding proteins (G-proteins) play important roles in signal transduction pathways in eukaryotic cells. Through differential screening of a hairy root cDNA library of tobacco (Nicotiana tabacum L.) against transcripts from non-root tissues of normal cuttings, we obtained a partial cDNA clone that showed abundant expression and high homology to the alpha subunit gene of plant G-protein. After RACE-PCR, a full-length cDNA clone was obtained, which was 1,677-bp in length and contained an open reading frame encoding a protein of 384 amino acids. A cDNA clone encoding a beta subunit of G-protein was also isolated from the same cDNA library based on PCR amplification and library screening. The clone was 1,600-bp in length and contained an open reading frame encoding 377 amino acids. The deduced amino acid sequences of these clones showed high homology (75.5 to 99.8% amino acid identity) with alpha and beta subunits of other plant G-proteins. Genomic Southern blot analysis showed that the amphidiploid tobacco genome possessed two major copies of both alpha and beta subunit genes and some minor homologous copies. Northern blot analysis showed that the transcript of a subunit gene was abundant in the root tissues, particularly in the hairy root tissues. In contrast, the level of expression of the beta subunit gene was equivalent in all the tissues studied. Possible function of tobacco G-protein was discussed.