Scaffold proteins and the regeneration of visual pigments

CRALBP, cellular retinaldehyde-binding protein, is a retinoid-binding protein necessary for efficient regeneration of rod and cone visual pigments. The C terminus of CRALBP binds to the PDZ domains of EBP50/NHERF-1, which in turn bind to ezrin and actin, proteins localized to the apical processes of the retinal pigment epithelium. In this study, we examined structural features associated with the interaction of the two proteins. The C-terminal amino-acid sequence of 11 orthologous CRALBPs is either ENTAL, ENTAF or EDTAL. Peptides ending in each of these sequences inhibited the interaction of CRALBP and EBP50/NHERF-1 with the use of an overlay assay. Molecular modeling showed that both NTAL and NTAF formed similar networks of H bonds with PDZ1 of EBP50/ NHERF-1, and the side chains of both C-terminal Leu and Phe fit into the peptide-binding groove of PDZ1x CRALBP.11-cis-retinal and EBP50/NHERF-1 migrated as single components when analyzed individually by gel filtration and as a complex when mixed together before gel filtration. Complex formation was abolished by preincubation of EBP50/NHERF-1 with peptide EVENTAL. The ligand absorption spectrum of the complex was identical with that of CRALBP x 11-cis-retinal, demonstrating that complex formation did not perturb the ligand-binding domain of CRALBP.