Glutamic acid rich protein (GARP) is a soluble protein localized to the outer segments of the rod photoreceptor. It forms a subunit of cyclic nucleotide-gated (CNG) channels, nonselective cation channels, which play important roles in both visual and olfactory signal transduction. When associated with CNGA1, it is involved in the regulation of ion flow into the rod photoreceptor outer segment (ROS), in response to light-induced alteration of the levels of intracellular cGMP. There are 3 isoforms produced by alternative splicing.
Isoform GARP2 is a high affinity rod photoreceptor phosphodiesterase (PDE6)-binding protein that modulates its catalytic properties; it is a regulator of spontaneous activation of rod PDE6, thereby serving to lower rod photoreceptor 'dark noise' and allowing these sensory cells to operate at the single photon detection limit.
Defects in GARP are the cause of retinitis pigmentosa type 25 (RP25). RP leads to degeneration of retinal photoreceptor cells. Patients typically have night vision blindness and loss of midperipheral visual field. As their condition progresses, they lose their far peripheral visual field and eventually central vision as well.