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Abstract
Guanylyl cyclase activating protein (GCAP1) has been proposed to act as a calcium-dependent regulator of retinal photoreceptor guanylyl cyclase (GC) activity. Using immunocytochemical and biochemical methods, we show here that GCAP1 is present in rod and cone photoreceptor outer segments where phototransduction occurs. Recombinant and native GCAP1 activate recombinant human retGC (outer segment-specific GC) and endogenous GC(s) in rod outer segment (ROS) membranes at low calcium. In addition, we isolate and clone a retinal homolog, termed GCAP2, that shows approximately 50% identity with GCAP1. Like GCAP1, GCAP2 activates photoreceptor GC in a calcium-dependent manner. Both GCAP1 and GCAP2 presumably act on GCs by a similar mechanism; however, GCAP1 specifically localizes to photoreceptor outer segments, while in these experiments GCAP2 was isolated from extracts of retina but not ROS. These results demonstrate that GCAP1 is an activator of ROS GC, while the finding of a second activator, GCAP2, suggests that a similar mechanism of GC regulation may be present in outer segments, other subcellular compartments of the photoreceptor, or other cell types.
Highlights
Guanylyl cyclase activating protein (GCAP1) has been proposed to act as a calcium-dependent regulator of retinal photoreceptor guanylyl cyclase (GC) activity
Like GCAP1, GCAP2 activates photoreceptor GCin a calcium-dependent manner. Both GCAP1 and GCAP2 presumably act on GCs by a similar mechanism; GCAP1 localizes to photoreceptor outer segments, while in these experiments GCAP2 was isolated from extracts of retina but not rod outer segment (ROS).These results demonstrate that GCAP1 is an activator of ROS GC, while the finding of a second activator, GCAP2, suggests that a similar mechanism of GC regulation may be present in outer segments, other subcellular compartments of the photoreceptor, or other cell types
GCAP1 and GCAP2 have almost identical calcium sensitivity in stimulating ROS GC (Fig. 8C). These results suggest that most likely GCAP1 and GCAP2 bind to the same sites in ROS membranes
Summary
Like GCAP1, GCAP2 activates photoreceptor GCin a calcium-dependent manner Both GCAP1 and GCAP2 presumably act on GCs by a similar mechanism; GCAP1 localizes to photoreceptor outer segments, while in these experiments GCAP2 was isolated from extracts of retina but not ROS.These results demonstrate that GCAP1 is an activator of ROS GC, while the finding of a second activator, GCAP2, suggests that a similar mechanism of GC regulation may be present in outer segments, other subcellular compartments of the photoreceptor, or other cell types. We proposed that a calcium binding protein isolated from rod outer segments (ROS), guanylyl cyclase activating protein (GCAP, termed here GCAP1), mediates this process (Gorczyca et al, 1994a), and its molecular properties were described (Palczewski et al.,1994). We show that the retina contains a second GCactivator, GCAP2, that is identical with p24 and closely related to GCAP1
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