부산대학교 도서관

The interaction of the GDP-bound form of the α-subunit of transducin (TαGDP) with the cGMP-specific phosphodiesterase, the effector enzyme in the visual system, has been studied. TαGDP is demonstrated to be able to activate the phosphodiesterase: (a) the basal activity in suspensions of dark-adapted retinal rod outer segments, examined in the absence of GTP, was found to be inhibited by binding of transducin to activated rhodopsin (Rh*) and by the complex of the β-and γ-subunits of transducin (Tβγ); (c) purified TαGDP is able to activate phosphodiesterase in the presence of membranes; (c) no activation is obtained either with holotransducin (TαGDPTβγ) or with TαGDP in the presence of excess Tβγ to prevent dissociation of TGDP. The maximal level of phosphodiesterase activation reached wit TαGDP (about 1500 mol cGMP/mol phosphodiesterase-1 · s-1) is similar to that obtained through the 'classical' activation by TαGDP whereas the apparent affinity of TαGDP for phosphodiesterase (Kd about 50 µM) is much lower than that of TαGDP. Our data suggest that GTP hydrolysis itself does not inactivate Tα. The role of Tβγ to sequester Tα is therefore of critical importance for phosphodiesterase inactivation. Our results support observations on the regulation of adenylyl cyclase by G-proteins, which suggested the ability of the free α-subunits loaded with GDP to activate their effectors. [ABSTRACT FROM AUTHOR]