THE REGULATION OF THE CGMP-BINDING CGMP PHOSPHODIESTERASE BY PROTEINSTHAT ARE IMMUNOLOGICALLY RELATED TO GAMMA-SUBUNIT OF THE PHOTORECEPTOR CGMP PHOSPHODIESTERASE

The cGMP phosphodiesterase from retinal rods (PDE-6) is an alpha beta gamma(2) heterotetramer, The alpha and beta subunits contain catalytic sites for cGMP hydrolysis, whereas the gamma subunits serve as a prot ein inhibitor of the enzyme, Visual excitation of photoreceptors enabl es the activated GTP-bound form of the G-protein transducin to remove the inhibitory action of the gamma subunit, thereby triggering PDE-B a ctivation, The type 5 phosphodiesterase (PDE-B) isoform shares a numbe r of similar characteristics with PDE-6, including binding of cGMP to non catalytic sites, the cyclic nucleotide specificity, and inhibitor sensitivities, Although the functional role of PDE 5 remains unclear, it has been shown to be activated by protein kinase A (PKA) (Burns, F. , Rodger, I. W. & Pyne, N. J. (1992) Biochem, J. 283, 487-491), Here w e report that both the recombinant gamma subunit and a peptide corresp onding to amino acids 24-46 in this protein inhibited the activation o f PDE-5 by PKA, Furthermore, immunoblotting airway smooth muscle membr anes with a specific antibody against amino acids 24-46 of the PDE-6 g amma subunit identified two major immunoreactive small molecular mass proteins of 14 and 18 kDa (p14 and p18), These appear to form a comple x with PDE-5, because PDE activity was immunoprecipitated using antibo dy against the PDE-6 gamma subunit, p14 and p18 were also substrates f or phosphorylation by a unidentified kinase that was stimulated by a p ertussis toxin-sensitive G-protein, Phosphorylation of p14/p18 in memb ranes treated with guanine nucleotides correlated with a concurrent re duction in the activation of PDE-B by PKA, We suggest that p14 and p18 share an epitope common to PDE-B gamma and that this region may inter act with PDE-6 to prevent its activation by PKA.