This lecture details the elucidation of cGMP
phosphodiesterase (PDEδ), discovered 25 years ago by Joe Beavo at the University of Washington. PDEδ, once identified as a fourth PDE6 subunit, is now regarded as a promiscuous
prenyl-
binding protein and important chaperone of prenylated
small G proteins of the Ras superfamily and prenylated
proteins of phototransduction. Alfred Wittinghofer's group in Germany showed that PDEδ forms an
immunoglobulin-like β-sandwich fold that is closely related in structure to other
lipid-
binding proteins, for example, Uncoordinated 119 (UNC119) and
RhoGDI. His group cocrystallized PDEδ with ARL (Arf-like) 2(
GTP), and later with farnesylated Rheb (ras homolog expressed in brain). PDEδ specifically accommodates farnesyl and geranylgeranyl moieties in the absence of bound
protein. Germline deletion of the Pde6d gene encoding PDEδ impeded transport of
rhodopsin kinase (GRK1) and PDE6 to outer segments, causing slowly progressing, recessive
retinitis pigmentosa. A rare PDE6D null allele in human patients, discovered by Tania Attié-Bitach in France, specifically impeded trafficking of farnesylated
phosphatidylinositol 3,4,5-trisphosphate (PIP3) 5-phosphatase (INPP5E) to cilia, causing severe syndromic
ciliopathy (
Joubert syndrome). Binding of cargo to PDEδ is controlled by
Arf-like proteins, ARL2 and ARL3, charged with guanosine-5'-triphosphate (
GTP).
Arf-like proteins 2 and 3 are unprenylated
small GTPases that serve as cargo displacement factors. The lifetime of ARL3(
GTP) is controlled by its
GTPase-activating protein,
retinitis pigmentosa protein 2 (RP2), which accelerates
GTPase activity up to 90,000-fold. RP2 null alleles in human patients are associated with severe X-linked
retinitis pigmentosa (XLRP). Germline deletion of RP2 in mouse, however, causes only a mild form of XLRP. Absence of RP2 prolongs the activity of ARL3(
GTP) that, in turn, impedes PDE6δ-cargo interactions and trafficking of prenylated
protein to the outer segments. Hyperactive ARL3(
GTP), acting as a hyperactive cargo displacement factor, is predicted to be key in the pathobiology of RP2-XLRP.