CALCIUM MODULATION OF BOVINE PHOTORECEPTOR GUANYLATE-CYCLASE

Bovine photoreceptor guanylate cyclase (ROS-GC) consists of a single t ransmembrane polypeptide chain with extracellular and intracellular do mains. In contrast to non-photoreceptor guanylate cyclases (GCs) which are activated by hormone peptides, ROS-GC is modulated in low Ca2+ by calmodulin-like Ca2+-binding proteins termed GCAPs (guanylate cyclase -activating proteins). In this communication we show that, like the na tive system, ROS-GC expressed in COS cells is activated 4-6-fold by re combinant GCAP1 at 10 nM Ca2+ and that the reconstituted system is inh ibited at physiological levels of Ca2+ (1 mu M). A mutant ROS-GC in wh ich the extracellular domain was deleted was stimulated by GCAP1 indis tinguishable from native ROS-GC indicating that this domain is not inv olved in Ca2+ modulation. Deletion of the intracellular kinase-like do main diminished the stimulation by GCAP1, indicating that this domain is at least in part involved in Ca2+ modulation. Replacement of the ca talytic domain in a non-photoreceptor GC by the catalytic domain of RO S-GC yielded a chimeric GC hat was sensitive to ANF/ATP and to a lesse r extent to GCAP1. The results establish that GCAP1 acts at an intrace llular domain, suggesting a mechanism of photoreceptor GC stimulation fundamentally distinct from hormone peptide stimulation of other cycla se receptors.