Differential Ca2+ Sensor Guanylate Cyclase Activating Protein Modes of Photoreceptor Rod Outer Segment Membrane Guanylate Cyclase Signaling

Photoreceptor ROS-GC1 (rod outer segment membrane guanylate cyclase) is a vital component of phototransduction. It is a bimodal Ca2+ signal transduction switch, operating between 20 and similar to 1000 nM. Modulated by Ca2+ sensors guanylate cyclase activating proteins 1 and 2 (GCAP1 and GCAP2, respectively), decreasing [Ca2+]; from 200 to 20 nM progressively turns it on, as does the modulation by the Ca2+ sensor S100B, increasing [Ca2+](i) from 100 to 1000 nM. The GCAP mode plays a vital role in phototransduction in both rods and cones and the S100B mode in the transmission of neural signals to cone ON-bipolar cells. Through a programmed domain deletion, expression, in vivo fluorescence spectroscopy, and in vitro reconstitution experiments, this study demonstrates that the biochemical mechanisms modulated by two GCAPs in Ca2+ signaling of ROS-GC1 activity are totally different. (1) They involve different structural domains of ROS-GC1. (2) Their signal migratory pathways are opposite: GCAP1 downstream and GCAP2 upstream. (3) Importantly, the isolated catalytic domain, translating the GCAP-modulated Ca2+ signal into the generation of cyclic GMP, in vivo, exists as a homodimer, the two subunits existing in an antiparallel conformation. Furthermore, the findings demonstrate that the N-terminally placed signaling helix domain is not required for the catalytic domain's dimeric state. The upstream GCAP2-modulated pathway is the first of its kind to be observed for any member of the membrane guanylate cyclase family. It defines a new model of Ca2+ signal transduction.