The GUCA1A gene encodes the guanylate cyclase activating protein 1 (GCAP1) of mammalian rod and cone photoreceptor cells, which is involved in the Ca2+-dependent negative feedback regulation of membrane bound guanylate cyclases in the retina. Mutations in the GUCA1A gene have been associated with different forms of cone dystrophies leading to impaired cone vision and retinal degeneration. Here we report the identification of three novel and one previously detected GUCA1A mutations: c.265G>A (p.Glu89Lys), c.300T>A (p.Asp100Glu), c.476G>T (p.Gly159Val) and c.451C>T (p.Leu151Phe). The clinical data of the patients carrying these mutations were compared with the functional consequences of the mutant GCAP1 forms. For this purpose we purified the heterologously expressed GCAP1 forms and investigated whether the mutations affected the Ca2+-triggered conformational changes and the apparent interaction affinity with the membrane bound guanylate cyclase. Furthermore, we analyzed Ca2+-dependent regulatory modes of wildtype and mutant GCAP1 forms. Although all novel mutants were able to act as a Ca2+-sensor protein, they differed in their Ca2+-dependent activation profiles leading to a persistent stimulation of guanylate cyclase activities at physiological intracellular Ca2+ concentration.