Comparison of the phosphodiesterase inhibitor subunit interactions of frog and bovine rod outer segments

Abstract

The rod outer segments of the bovine and frog retina possess a cyclic GMP phosphodiesterase (PDE) that is composed of two larger subunits, α and β (Pαβ), which contain the catalytic activity and a smaller γ (Pγ) subunit which inhibits the catalytic activity. We studied the binding of Pγ to Pαβ in both the bovine and frog rod outer segment membranes. Analysis of these data indicates that there are two classes of Pγ binding sites per Pαβ in both species. The activation of PDE by the guanosine 5-[γ-thio]triphosphate form of the α subunit of transducin, Tα·GTPγS, was also studied. These data indicate that the two classes of Pγ binding sites contribute to the formation of two classes of binding sites for Tα·GTPγS. We demonstrate solubilization of a portion of the Pγ by Tα·GTPγS in both species. There is also present, in both species, a second class of Pγ which is not solubilized even when it is dissociated from its inhibitory site on Pαβ by Tα·GTPγS. The amount of full PDE activity which results from release of the solubilizable Pγ is about 50% in the frog PDE but only approx. 175 in the bovine PDE. We also show that activation of frog rod outer segement PDE by trypsin treatment releases the PDE from the membranes. This type of release by trypsin has already been demonstrated in bovine rod outer segments [Wensel & Stryer (1986) Proteins: Struct. Funct. Genet. 1, 90-99].