Molecular Dissection of Multifunctional Proteins in Rod Outer Segments

Rod photoreceptors are specialized neurons responsible for capturing photons and translating visual information into electrical signals. Visual signal transduction in rods is confined to the unique outer segment organelle, a modified primary cilium consisting of a stack of hundreds of flattened disc membranes enveloped by a single plasma membrane. By concentrating important signaling molecules on disc membranes, the outer segment provides an ideal biochemical environment for the production of vision with high sensitivity and temporal resolution.

This dissertation focuses primarily on a molecular dissection of two multifunctional outer segment proteins, R9AP and rhodopsin, and also reassesses the localization of Glut1, a third protein formerly believed to reside in the outer segment. All three experimental lines relied on in vivo expression of novel protein constructs in vertebrate rods using several gene delivery strategies: conventional transgenics, retinal electroporation, and retinal infection with recombinant adeno-associated virus.

The tail-anchored protein R9AP, in conjunction with RGS9-1 and G-beta5, comprises the transducin GTPase activating complex, which catalyzes the rate-limiting step in rod photoresponse recovery. In addition to maximizing the enzymatic activity of the complex, R9AP is responsible for both the post-translational stability and outer segment targeting of RGS9-1-G-beta5. We investigated the mechanism behind R9AP's poorly understood function in protecting RGS9-1-G-beta5 from proteolysis and found that it is performed simply by recruiting the complex to cellular membranes and can be entirely dissociated from R9AP's outer segment targeting function. Furthermore, we demonstrated that replacement of R9AP's transmembrane domain with a lipid anchor preserves the ability of the GTPase activating complex to function in outer segments.

Rhodopsin, the visual pigment of rods, has a second important, yet poorly defined, function as a rod outer segment building block: outer segments disc membranes fail to form in the absence of rhodopsin. Our goal was to identify the molecular features of rhodopsin mechanistically involved in outer segment morphogenesis by designing artificial membrane proteins that could fully substitute for rhodopsin in performing this function. We observed that rhodopsin's C-terminal VXPX outer segment targeting motif is unnecessary for outer segment disc formation since it could be replaced with a targeting motif from an unrelated protein, peripherin. Furthermore, we obtained surprising evidence that rhodopsin's role in this process is limited to providing an abundance of transmembrane protein material to disc membranes.

Finally, while attempting to find a targeting motif to substitute for the VXPX motif of rhodopsin, we made an unexpected observation that the facilitative glucose transporter Glut1, long thought to reside in the outer segment, is actively excluded from this organelle. This revises our understanding of the energy flow in rods by showing that the outer segment is entirely dependent on the inner segment for its energy supply.