MECHANISMS OF RHODOPSIN INACTIVATION IN-VIVO AS REVEALED BY A COOH-TERMINAL TRUNCATION MUTANT

Although biochemical experiments suggest that rhodopsin and other rece ptors coupled to heterotrimeric guanosine triphosphate-binding protein s (G proteins) are inactivated by phosphorylation near the carboxyl (C OOH)-terminus and the subsequent binding of a capping protein, little is known about the quenching process in vivo. Flash responses were rec orded from rods of transgenic mice in which a fraction of the rhodopsi n molecules lacked the COOH-terminal phosphorylation sites. In the sin gle photon regime, abnormally prolonged responses, attributed to activ ation of individual truncated rhodopsins, occurred interspersed with n ormal responses. The occurrence of the prolonged responses suggests th at phosphorylation is required for normal shutoff. Comparison of norma l and prolonged single photon responses indicated that rhodopsin begin s to be quenched before the peak of the electrical response and that q uenching limits the response amplitude.