Identification of an ARF type low molecular mass GTP-binding protein in pea (Pisum sativum)

The ARF class of low molecular mass GTP-binding proteins, originally detected as cholera toxin-activated ADP-ribosylation factors (ARF) in mammalian cells, is now known to participate in intracellular membrane vesicle trafficking. We identified a GTP-binding protein in pea plumules which resembles ARF in several respects. Like mammalian ARF, the pea protein had an apparent molecular mass of 21 kDa and was distributed mainly (approximately 97%) in the cytosol, with 1% and 1.5% occurring in the Golgi and microsomal fractions, respectively. In comparison, small GTP-binding proteins in the 26-30 kDa range were enriched in membranous fractions. The 21 kDa protein crossreacted strongly with an ARF class I antibody prepared against a mammalian ARF, but did not crossreact with ARF 5 (of class II) antibodies. An anti-Volvox yptl antibody did not show immunoreactivity to the 21 kDa pea protein, although it crossreacted strongly with a 28 kDa pea plumule protein. Our results strongly suggest that the 21 kDa pea protein is an analog of the ARF protein localized in the cytosol of mammalian cells. As far as we are aware, this is the first observation of an ARF protein in plants.