Domain-Specific Stabilization of Photoreceptor Membrane Guanylyl Cyclase by Adenine Nucleotides and Guanylyl Cyclase Activating Proteins (GCAPs)

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Abstract
In photoreceptor outer segments, particulate guanylyl cyclase (RetGC) is stimulated at low intracellular Ca2+ concentrations by guanylyl cyclase activating protein (GCAP), a Ca2+-sensitive activator, to resynthesize light-depleted cGMP. In washed outer segment membranes, we find that GCAP-stimulable RetGC is rapidly inactivated at physiological temperatures (30-37 degrees C). Under the same conditions, RetGC remains competent for stimulation by S-100 protein preparations or Mn2+/Triton X-100, indicating that the cyclase catalytic domain remains functional. GCAPs and adenine nucleotides protect against inactivation. Protection by GCAPs is independent of Ca2+ concentration, suggesting that there is a Ca2+-independent interaction between GCAP and RetGC. Protection by ATP (EC50 = 150 microM) is not due to phosphorylation, since the nonhydrolyzable analogue adenylyl imidodiphosphate (AMP-PNP) protects equally well. In addition to their roles in protection, ATP and AMP-PNP also slowly stimulate cyclase activity. In parallel with the functional change in RetGC at physiological temperatures, we also observe a structural change. A 62-kDa intracellular fragment of RetGC-1 becomes more sensitive to cleavage by trypsin after preincubation at 30 degrees C unless ATP, AMP-PNP, or GCAP is present. Adenine nucleotides and GCAPs thus protect RetGC structurally, as well as functionally.