R9AP targeting to rod outer segments is independent of rhodopsin and is guided by the SNARE homology domain.

In vertebrate photoreceptor cells, rapid recovery from light excitation is dependent on the RGS9⋅Gβ5 GTPase-activating complex located in the light-sensitive outer segment organelle. RGS9⋅Gβ5 is tethered to the outer segment membranes by its membrane anchor, R9AP. Recent studies indicated that RGS9⋅Gβ5 possesses targeting information that excludes it from the outer segment and that this information is overridden by association with R9AP, which allows outer segment targeting of the entire complex. It was also proposed that R9AP itself does not contain specific targeting information and instead is delivered to the outer segment in the same post-Golgi vesicles as rhodopsin, because they are the most abundant transport vesicles in photoreceptor cells. In this study, we revisited this concept by analyzing R9AP targeting in rods of wild-type and rhodopsin-knockout mice. We found that the R9AP targeting mechanism does not require the presence of rhodopsin and further demonstrated that R9AP is actively targeted in rods by its SNARE homology domain.