Interaction of Transducin with Retinal cGMP Phosphodiesterase

Abstract

Transducin is a member of the large family of heterotrimeric G-proteins that convey hormonal or sensory signals from transmembranous receptors to various types of effector proteins. It is a peripheral membrane protein, bound to the cytoplasmic face of the membrane from which it can be detached easily by lowering the ionic strength of the surrounding medium (see Kühn 1984 for a review). The three polypeptides form only two functional subunits, T _α (39 kDa) and T _βγ (36 + 6 kDa), as T _β and T_γ cannotbe dissociated without denaturation. T _α bearsa guanine nucleotide binding site that is highly conserved among all G-proteins. In the resting state, i.e., in the absence of photoexcited rhodopsin, a GDP, practically inexchangeable on a time scale of hours, remains locked in this nucleotide site (Bennett and Dupont 1985). T _α GDP, associated to T _βγ diffuses freely on the membrane surface. In the “activated state”, i.e., with a bound GTP, T _α GTP is dissociated from T _βγ . The β-polypeptide of transducin is strictly identical to the 36 kDa β-polypeptide of Gs or Gi. Differences between the γ-polypeptides must therefore account for the fact that isolated T _βγ is only weakly membrane attached and partially soluble even in a physiological medium, while the other G _βγ appear very strongly membrane attached.