The immunolocalization and divergent roles of phosducin and phosducin-like protein in the retina

Purpose: These investigations were undertaken to compare and contrast the roles of phosducin and phosducin-like protein in the retina. Methods: Phosducin and phosducin-like protein were compared in an in vitro assay measuring their inhibition of transducin binding to light-activated rhodopsin. The two proteins were localized within the retina by immunoblot analyses and immunocytochemistry using affinity-purified antibodies with high specificity for each of the two homologs. The sensitivity of phosducin-like protein to phosphorylation was probed using in vitro protein kinase reactions. Results: Phosducin and phosducin-like protein were found to have similar, though not identical, transducin inhibiting activity in vitro. These two proteins were found to be localized dissimilarly within the retina, with spatial overlap limited to the inner segments of the photoreceptors. Phosducin is found exclusively in photoreceptor cells, including the synaptic and nuclear layers, while phosducin-like protein is found throughout the inner retinal layers, most abundantly in the bipolar cells of the inner nuclear layer. Phosducin-like protein is not efficiently phosphorylated by the protein kinases tested, indicating that its regulation differs from that of phosducin. Conclusions: It appears that phosducin and phosducin-like protein play distinct roles in the retina. While phosducin is likely to be important in feedback regulation of the visual signal, such as in light adaptation, phosducin-like protein probably has little if any function in the phototransduction cascade. Phosducin-like protein may have a role in regulating the processing of visual signals by the neural cells of the inner retina.