G-PROTEIN FROM MEDICAGO-SATIVA - FUNCTIONAL ASSOCIATION TO PHOTORECEPTORS

G-protein subunits were characterized from Medicago sativa (alfalfa) s eedlings. Crude membranes and GTP-Sepharose-purified fractions were el ectrophoresed on SDS/polyacrylamide gels and analysed by Western blott ing with 9193 (anti-alpha(common)) and AS/7 (anti-alpha(t), anti-alpha (i1) and anti-alpha(i2)) polyclonal antibodies. These procedures led t o the identification of a specific polypeptide band of about 43 kDa. A nother polypeptide reacting with the SW/1 (anti-beta) antibody, of abo ut 37 kDa, was also detected. The 43 kDa polypeptide bound specificall y [alpha-P-32]GTP by a photoaffinity reaction and was ADP-ribosylated by activated cholera toxin, but not by pertussis toxin. Irradiation of etiolated Medicago sativa protoplast preparations at 660 nm for 1 min produced a maximal increase in the guanosine 5'-[gamma-thio]triphosph ate (GTP[S-35])-binding rate. After this period of irradiation, the bi nding rate tended to decrease. The effect of a red-light (660 nm) puls e on the binding rate was reversed when it was immediately followed by a period of far-red (> 730 nm) illumination. These results may sugges t that activation of GTP[S]-binding rate was a consequence of conversi on of phytochrome P(r) into the P(fr) form.