CONSTITUTIVE ACTIVATION OF PHOTORECEPTOR GUANYLATE-CYCLASE BY Y99C MUTANT OF GCAP-1 - POSSIBLE ROLE IN CAUSING HUMAN AUTOSOMAL-DOMINANT CONE DEGENERATION

Photoreceptor membrane guanylate cyclases (RetGC) are regulated by cal cium-binding proteins, GCAP-1 and GCAP-2. At Ca2+ concentrations below 100 nM, characteristic of light adapted photoreceptors, guanylate cyc lase-activating protein (GCAPs) activate RetGC, and at free Ca2+ conce ntrations above 500 nM, characteristic of dark-adapted photoreceptors, GCAPs inhibit RetGC. A mutation, Y99C, in human GCAP-1 was recently f ound to be linked to autosomal dominant cone dystrophy in a British fa mily (Payne, A. M., Downes, S. M., Bessant, D. A. R., Taylor, R., Hold er, G. E., Warren, M. J., Bird, A. C., and Bhattachraya, S. S. (1998) Hum. Mol. Genet. 7, 273-277). We produced recombinant Y99C GCAP-1 muta nt and tested its ability to activate RetGC in vitro at various free C a2+ concentrations. The Y99C mutation does not decrease the ability of GCAP-1 to activate RetGC. However, RetGC stimulated by the Y99C GCAP- 1 remains active even at Ca2+ concentration above 1 mu M. Hence, the c yclase becomes constitutively active within the whole physiologically relevant range of free Ca2+ concentrations. We have also found that th e Y99C GCAP-1 can activate RetGc even in the presence of Ca2+ loaded n onmutant GCAPs. This is consistent with the fact that cone degeneratio n was dominant in human patients who carried such mutation (Payne, A. M., Downes, S. M., Bessant, D. A. R., Taylor, R., Holder, G. E., Warre n, M. J., Bird, A. C., and Bhattachraya, S. S. (1998) Hum. Mol. Genet. 7, 273-277). A similar mutation, Y104C, in GCAP-2 results in a differ ent phenotype. This mutation apparently does not affect Ca2+ sensitivi ty of GCAP-S. Instead, the Y104C GCAP-2 stimulates RetGC less efficien tly than the wild-type GCAP-2. Our data indicate that cone degeneratio n associated with the Y99C mutation in GCAP-1 can be a result of const itutive acti vation of cGMP synthesis.