is rod photoreceptor degeneration from RP, often caused by rhodopsin mutations. Non-RP mutations generating constitutive receptor activation cause CSNB. Rhodopsin is constrained in an inactive conformation through the salt-bridge interaction between lysine-296 and glutamate-113 [84]; mutation of either component can generate constitutive activity, causing CSNB. Interestingly, one of the earliest symptoms of RP is night blindness [85], demonstrating a fascinating relationship between rhodopsin folding and activity. RP mutation K296E, however, has been shown to cause photoreceptor degeneration through a process independent of constitutive activity [86]. Proximity of glycine-90 to this counterion permits G90D interference of the K296-E113 salt bridge by substituting for E113 in salt-bridge formation, leading to constitutive activation and CSNB [87]. The T94I autosomal dominant CSNB mutation is similarly near the G90 region and also results in constitutive transducin activation, most likely through hydrophobic interference or steric hindrance [88].

Cytoplasmic RP Rhodopsin Mutants

The cytoplasmic region of rhodopsin extends from the surface of disks into the cellular milieu, where transducin (Gt) binding, and hence signaling, occurs. This intracellular region consists of the first cytoplasmic loop connecting helices 1 and 2, the second cytoplasmic loop connecting helices 3 and 4, the third cytoplasmic loop, and the C-terminal tail. The C-terminus is anchored to the membrane surface by palmitoylation of residues Cys322 and Cys323, creating what is putatively referred to as the fourth cytoplasmic loop [89, 90]. The proximal portion of this loop forms a part of the binding site for the C-terminal section of the transducin α-subunit [91]. RP mutations within the cytoplasmic region of rhodopsin are found throughout the region, with the exception of the third cytoplasmic loop (Fig. 4C). The C-terminus directly interacts with a cargo-binding subunit of dynein, providing transport of rhodopsin to the outer segment [92], an interaction abolished by severe RP mutants P347L, P347S, V345M, and Q344ter. C-terminal tail also contains multiple phosphorylation sites, phosphorylated by RK following transducin activation. Phosphorylation of the C-terminal tail regulates interaction of rhodopsin with arrestin, deactivating the receptor. These multiple phosphorylation sites result in uniquely consistent control of the amplitude and duration of the activated rhodopsin signal [40].

Intradiskal RP Rhodopsin Mutants

Structural investigations of the RP mutants and their consequences on the intradiskal region were best highlighted in a series of investigations by Khorana et al. [23, 25, 26, 93–95]. Investigations by this group established a number of techniques critical for furthering rhodopsin research, while demonstrating important general principles of rhodopsin structure and the misfolding properties of numerous RP mutants. Together, their findings suggest that packing of the transmembrane helices, binding of the chromaphore, and intradiskal structural integrity are physically coupled properties critical for proper folding of rhodopsin. Intradiskal RP mutations are numerous and diverse in their impact on misfolding (Fig. 4D).

Deletion studies of the intradiskal region provided suggestions for the structural importance of this region [96]. N-terminal deletions resulted in partial chromophore