DIFFERENTIAL ACTIVATION OF ROD OUTER SEGMENT MEMBRANE GUANYLATE CYCLASES, ROS-GC1 AND ROS-GC2, BY CD-GCAP AND IDENTIFICATION OF THE SIGNALING DOMAIN

The ROS-GC is one of the two subfamilies of membrane guanylate cyclase s, It distinguishes itself from the other surface receptor subfamily i n that its members are not regulated by extracellular peptides; instea d, they are modulated by intracellular Ca2+ signals. There are two mem bers of the subfamily, ROS-GC1 and ROS-GC2. An intriguing feature of R OS-GC1 is that it has two Ca2+ switches. One switch inhibits the enzym e at micromolar concentrations of Ca2+, and the other stimulates. The inhibitory switch is Linked to phototransduction, and it is likely tha t the stimulatory switch is linked to retinal synaptic activity. Ca2acts indirectly via Ca2+-binding proteins, GCAPs and CD-GCAP. GCAPs mo dulate the inhibitory switching component of the cyclase and CD-GCAP t urns on the activation signaling switch, The activating switch of ROS- GC2 has not so far been scrutinized. The present study shows that CD-G CAP is linked to the activation signaling switch of ROS-GC2, but the l inkage is about 10-fold weaker than that of the ROS-GC1. Thus, CD-GCAP is a specific ROS-GC1 activator, Furthermore, through a series of exp ression studies on the mutants involving deletion, building of hybrids , and reconstruction of a heterologous cyclase, the study confirms tha t the CD-GCAP regulated switch resides within the amino acid segment 7 36-1053 of the cyclase. (C) 1998 Academic Press.