| Abstrakt: |
There is considerable evidence which suggests that the γ-subunit of cGMP phosphodiesterase (PDEγ) is a multifunctional protein which may interact directly with both the catalytic subunits of PDE (PDEαβ) and the α-subunit of transducin (Tα) (Whalen, M., and Bitensky, M. (1989) Biochem. J.259, 13–19; Griswold-Prenner, I., Young, J. H., Yamane, H. K., and Fung, B. K.-K. (1988) Invest. Ophthalmol. & Visual Sci.29, (Suppl.) 218). To determine the region of interaction between the multifunctional PDEγ and Tα, and to determine the significance of this interaction, peptides corresponding to various regions of PDEγ were synthesized and tested for their ability to inhibit the GTPase activity of Tα. One of these peptides, PDEγ-3 (bovine amino acid residues 31–45), inhibited the GTPase activity of Tα with an I50of 450 µM. The peptide (PDEγ-3) was found to inhibit the GTPase activity of Tα by inducing the binding of transducin to the rod outer segment membrane and by altering the GTP/GDP exchange. Analogs of PDEγ-3 were synthesized to determine the required structure of the PDEγ-3 region needed for the interaction of PDEγ with Tα. The results of these studies indicated that the removal of the positively charged amino acids or any of the potential hydrogen-bonding amino acids increased the I50for the inhibition of the GTPase activity of Tα Substitution of the hydrophobic amino acids had no effect. These results indicate the hydrophilic interactions may be essential for the binding of PDEγ to Tα and for the inhibition of the GTPase activity of Tα by PDEγ. The observed effects of PDEγ-3 on Tα and on PDE suggest that PDEγ is a multifunctional protein which may play more than one role in the deactivation of the retinal transduction cascade. |
