Biophysical and biochemical characterization of two novel GCAP1 mutants associated with cone-rod dystrophy

Purpose Guanylate Cyclase Activating Protein 1 (GCAP1) is a Ca2+-sensor protein involved in the regulation of Guanylate Cyclase (GC) during the phototransduction cascade, which initiates the visual process. An increasing number of GCAP1 mutants has been found to be associated with degenerative retinal diseases such as cone dystrophy (COD) and cone-rod dystrophy (CORD). This study is focused on the structural and functional characterization of two novel CORD-associated GCAP1 mutants, namely I107T and L84F. Methods Circular dichroism spectroscopy was employed to investigate changes in protein secondary and tertiary structure and in thermal stability both in the absence and in the presence of physiological 1 mM Magnesium (Mg2+) and saturating Ca2+ concentration. Variations in hydrodynamic radius of the mutants in the aforementioned conditions was monitored by dynamic light scattering. Ca2+-binding constants were estimated by a chromophoric chelator assay. The conformational transition range upon Mg2+ or Ca2+ binding was investigated by monitoring the tryptophan fluorescence in titration experiments. Results I107T-GCAP1 exhibited similarities with the wild type in terms of conformational and hydrodynamic radius changes upon Mg2+ or Ca2+ binding, while its Ca2+ affinity was severely impaired and its stability was increased independently on the presence of Ca2+ . Ca2+ fluorescence titrations showed a biphasic pattern similar to the COD-associated G159V mutant. L84F-GCAP1 showed structural features significantly different from the wild type, with small differences in secondary structure but major differences in tertiary structure upon Ca2+ binding. Moreover this mutant showed higher thermal stability than the wild type particularly in the presence of Ca2+ and appeared to be oligomeric both in the presence and in the absence of Ca2+ or Mg2+. Conclusion Our results suggest that these two novel CORD-associated GCAP1 mutants could affect GC regulation via different processes. Indeed I107T-GCAP1 might alter the Ca2+ regulation of GC by its impaired Ca2+-sensitivity, while L84F-GCAP1 may lead to different supramolecular assemblies as a consequence of its oligomeric state.