SUBUNIT STOICHIOMETRY OF RETINAL ROD CGMP PHOSPHODIESTERASE

The cyclic GMP phosphodiesterase of the retinal rod is composed of three distinct types of polypeptides: α (90 kDa), ß (86 kDa), and γ (10 kDa). The y subunit has been shown to inhibit phosphodiesterase activity associated with α and ß. To investigate the subunit stoichiometry of the retinal phosphodiesterase, we have developed a panel of monoclonal and peptide antibodies that recognize individual phosphodiesterase subunits. By quantitative and immunochemical analysis of the purified subunits, we have shown that each phosphodiesterase molecule contains one copy each of α and ß subunit and two copies of γ subunit. Moreover, γ can be chemically cross-linked to both α and ß, but not to itself, suggesting that α and ß may each bind one γ. The phosphodiesterase is fully activated when both copies of γ were removed by proteolysis with trypsin. Upon recombination of the purified γ subunit with the trypsin-activated phosphodiesterase containing αß, the αßγ2 stoichiometry is once again restored, with concomitant total inhibition of activity. Our results suggest that at least two activated transducin molecules are required to fully activate one molecule of phosphodiesterase in retinal rods. © 1990, American Chemical Society. All rights reserved.