A new factor for cellular signal transduction-Studies of the structures and functions of soluble GTP binding proteins

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Incubation of the GH_4C_1 rat pituitary cells with somatostatin (SRIF) decreased the level of alpha subunit of G_<i2> to 52% of control values in the crude membrane during the first 8 min, whereas the 274,000 x g supernatant (cytosolic fraction) showed clearly the alpha subunit of G_<i2> by immunoblotting. The resolved cytosolic fraction was ADP-ribosylated by pertussis toxin (PT) with betagamma complex. The effects of SRIF were inhibited by the pretreatment of GH_4C_1 cells with PT. Furthermore, the addition of vasoactive intestinal peptide (VIP) to intact GH_4C_1 cells also decreased the level of alpha subunit (50 KDa) of G_s in crude membranes, whereas the cytosolic fraction revealed the alpha subunit of G_s the during first 30 min. These data indicate that PT-sensitive or cholera toxin-sensitive G proteins in intact GH_4C_1 cells do indeed dissociate into alpha subunit and betagamma complex upon SRIF or VIP activation and release alpha subunits from the membrane to cytosol.