| Popis: |
A guanine nucleotide-binding regulatory protein, called transducin or G t couples light-activated rhodopsin with the cGMP phosphodiesterase (Hurley, 1980). Like all G proteins, transducin is a heterotrimer composed of two distinct subunits: αt (39 kDa) and βγt (βt: 36 kDa, γt: 8 kDa). The αt subunit binds GDP and GTP (Fung and Stryer, 1980), and in its GTP-bound form activates the cGMP phosphodiesterase (Fung et al., 1981). The activation is terminated when the bound GTP is hydrolyzed to GDP by an intrinsic GTPase activity (Wheeler and Bitensky, 1977, Fung et al., 1981). Although the βγt subunit does not directly participate in either GTP hydrolysis or phosphodiesterase activation, its presence is important for effective binding of αt to photolyzed rhodopsin and GTP-GDP exchange (Fung, 1983). Transducin binds tightly to photolyzed rhodopsin in intact rod outer segment membranes (Kuhn, 1980). It also binds selectively to unphotolyzed rhodopsin in rod outer segment membranes (Hamm et al., 1987) and in reconstituted phospholipid vesicles (Fung, 1983). |
