A 48 kDa protein arrests cGMP phosphodiesterase activation in retinal rod disk membranes

AbstractPhotolyzed rhodopsin (R∗) catalyzes GTP-binding to α-transducins (Tα); Tα-GTPs then activate cGMP phosphodiesterase (PDE). PDE activation is arrested by ATP in two ways: (i) initial velocity is suppressed, and (ii) PDE velocity rapidly returns to preactivation levels (turnoff). Arrestin (a 48 kDa protein) markedly enhances turnoff while not affecting initial velocity. Arrestin in the presence of ATP achieves rapid turnoff by directly inhibiting activated PDE, as indicated by its ability to inhibit the direct activation of PDE by Tα-GMP-PNP (guanylyl-imidodiphosphate). Double reciprocal plots reveal a competition between arrestins and activated transducins for sites on PDE. Blocking R∗ phosphorylation blocks initial velocity suppression but does not disturb rapid turnoff. Our data suggest a 2-fold mechanism for PDE deactivation: (i) formation of Tα.GTPs is suppressed by R∗ phosphorylation, while (ii) activation of PDE by Tα·GTPs is competitively inhibited by arrestins when ATP is present