Protein kinase CK2 modulates the activity of Maf-family bZIP transcription factor NRL in rod photoreceptors of mammalian retina.

Abstract

Maf-family basic motif leucine zipper protein NRL specifies rod photoreceptor cell fate during retinal development and, in concert with homeodomain protein CRX and other regulatory factors, controls the expression of most rod-expressed genes including the visual pigment gene Rhodopsin (Rho). Transcriptional regulatory activity of NRL is modulated by post-translational modifications, especially phosphorylation, and mutations at specific phosphosites can lead to retinal degeneration. During our studies to elucidate NRL-mediated transcriptional regulation, we identified protein kinase CK2 in NRL-enriched complexes bound to Rho promoter-enhancer regions and in NRL-enriched high molecular mass fractions from the bovine retina. The presence of CK2 in NRL complexes was confirmed by co-immunoprecipitation from developing and adult mouse retinal extracts. In vitro kinase assay and bioinformatic analysis indicated phosphorylation of NRL at Ser117 residue by CK2. Co-transfection of Csnk2a1 cDNA encoding murine CK2 with human NRL and CRX reduced the bovine Rho promoter-driven luciferase expression in HEK293 cells and mutagenesis of NRL-Ser117 residue to Ala restored the reporter gene activity. In concordance, overexpression of CK2 in the mouse retina in vivo by electroporation resulted in reduction of Rho promoter-driven DsRed reporter expression as well as the transcript level of many phototransduction genes. Thus, our studies demonstrate that CK2 can phosphorylate Ser117 of NRL. Modulation of NRL activity by CK2 suggests intricate interdependence of transcriptional and signaling pathways in maintaining rod homeostasis.