PDE6B

, PDEA red: Rod cGMP-specific 3',5'-cyclic phosphodiesterase subunit beta, Gene names: PDE6B, PDEB blue: Retinal rod rhodopsin-sensitive cGMP 3',5'-cyclic phosphodiesterase subunit gamma, Gene names: PDE6G, PDEG PDE6 is a protein complex located on the photoreceptor's outer segment, and plays an important role in the phototransduction cascade. There are two types of photoreceptors: cones and rods. The rod and cone PDE6 complexes have different structures. PDE6β together with PDE6α and two identical inhibitory subunits, PDE6γ, form the rod PDE6 holoenzyme while the cone PDE6 complex only consists of two identical PDE6α' catalytic subunits. PDE6β, one of the catalytic units in rod PDE6, is composed of three domains: two N-terminal GAF domains and one C-terminal catalytic domain. The non-catalytic GAF domains are responsible for cGMP binding. The C-terminal interacts with cell membrane by isoprenylation and S-carboxylmethylation. == Function ==

Absorption of photons by rhodopsin triggers a signal transduction cascade in rod photoreceptors. This phototransduction cascade leads to hydrolysis of cGMP by cGMP-phosphodiesterase (PDE) that closes cGMP-gated channels and hyperpolarizes the cell. PDE6β is necessary for the formation of a functional phosphodiesterase holoenzyme. The geranylgeranyl group of PDE6B is the result of these modifications, which are responsible for the rod PDE6's interaction with membrane. The figure at left shows the PDE6 aalpha/beta dimer in blue and purple, with the gamma ubunits in green and orange. == Animal studies ==

rd1 mouse Mutation of the PDE6b gene leads to the dysfunction of PDE, which results in failure of hydrolysis of cGMP. The rd1 mouse is a well-characterized animal model of retinitis pigmentosa caused by the mutation of Pde6b gene. The phenotype was first discovered in rodless mice in the 1920s by Keeler. An insertion of Murine leukemia provirus is present near the first exon combined with a point mutation, which introduces a stop codon in exon 7. In addition to the rd1 mouse, a missense mutation (R560C) in exon 13 of the Pde6b gene is the character of another animal model of recessive retinal degeneration. In rd1 animals, the retinal rod photoreceptor cells begin degenerating at about postnatal day 10, and by 3 weeks no rod photoreceptors remain. Degeneration is preceded by accumulation of cGMP in the retina and is correlated with deficient activity of the rod photoreceptor cGMP-phosphodiesterase. Cone photoreceptors undergo a slower degeneration over the course of a year, which causes the mutants to completely go blind. The possibility of altering the course of retinal degeneration through subretinal injection of recombinant replication defective adenovirus that contained the murine cDNA for wildtype PDE6β was tested in rd1 mice. Subretinal injection of rd1 mice was carried out 4 days after birth, before the onset of rod photoreceptor degeneration. Following therapy, Pde6β transcripts and enzyme activity were detected, and histologic studies revealed that photoreceptor cell death was significantly retarded. == References ==