Fig. 7.

Model of light-dependent attenuation of Rho* by p44 and arrestin. In the dark-adapted rod disk (upper panel), p44 (light blue) is “tethered” to the membrane by its interaction with Rho-P (red), which comprises ~1% of the dark-state rhodopsin population (Binder et al., 1996). Transducin (G_t, blue, green, and orange subunits) and rhodopsin kinase (RK, purple) are also localized to the membrane by lipid membrane anchors (for simplicity, the interactions of RK with recoverin and G_βγ are ignored in this schematic). Upon exposure to dim light (middle panels), a single rhodopsin molecule (red) is converted to Meta II (yellow). (1) G_t binds Meta II and is activated. (2) Meta II is phosphorylated by RK. (3) p44 binds phosphorylated Meta II and blocks further G_t activation. It is not known exactly how p44 “hops” from Rho-P to Rho*-P, but rhodopsin dimerization may facilitate this process (Schröder et al., 2002). Furthermore, the rapid diffusion of rhodopsin monomeric units in the ROS membrane (Cone, 1972; Liebman & Entine, 1974) may allow p44-bound Rho-P to come into contact with Rho*-P, but this possibility is speculative. Note that different modes of p44 binding to Rho-P and Rho*-P are illustrated (see text for more details). Upon exposure to bright light (lowest panel), which results in complete rhodopsin bleaching, arrestin (dark blue) translocation from the inner segment might provide a pool of protein sufficient to bind up every photo-activated receptor (Strissel et al., 2006). In the figure, rhodopsin dimers and a binding stoichiometry of 2 arrestin to 2 Rho*-P are suggested. (For interpretation of the references to colors in this figure legend, the reader is referred to the web version of this paper.)