The C-terminal domain of RD3 enables accumulation of retinal membrane guanylyl cyclase (RetGC) in photoreceptor outer segment.

Retinal degeneration-3 protein (RD3) plays a dual role in photoreceptors - prevents their degeneration by suppressing aberrant activity of retinal membrane guanylyl cyclase (RetGC) in the inner segment and enables photoreceptor function by facilitating delivery of RetGC to the outer segment. Parts of RD3 structure supporting its dual function were evaluated in vivo using deletion mutants of a human RD3 transgenically expressed under the control of rod opsin promoter in Rd3-/- mouse rods lacking endogenous RD3. The human RD3 truncated after Gly148 or Arg158 not only inhibited RetGC activation by the guanylyl cyclase activating protein (GCAP) in vitro but also prevented rapid degeneration of the RD3-deficient rods in transgenic mice. However, these deletion mutants did not restore RetGC trafficking in Rd3-/- rods to the outer segment or normal rod function. Extending RD3 polypeptide to Ser170 restored RetGC accumulation in the outer segment of rescued Rd3-/- rods and enabled their photoresponse. These findings indicate that the RD3 serves as a 'ski lift' for RetGC produced in the inner segment, in which Arg158-Ser170 region of RD3 mediates coupling of the RetGC:RD3 complex to intracellular protein trafficking, while its α-helical core N-terminal to Gly148 binds RetGC and suppresses its aberrant activation by GCAP in the inner segment in order to prevent degeneration of photoreceptors.