LGN, a transducin– binding protein, is present in the inner segments and synaptic terminals of photoreceptor cells.

Abstract

Abstract: : Purpose: To identify potential photoreceptor proteins containing G–protein regulatory (GPR)–motifs. Methods: A Northern blot analysis of human retinal mRNA with probes corresponding to known and putative GPR–proteins identified through the search of gene and EST databases. A Western blot analysis of candidate retinal GPR–proteins. Localization of LGN in retina was investigated by immunoflurescence staining of cryosections of mouse retina. The interaction of LGN with transducin–α (Gtα) was examined using immunoprecipitation with anti–LGN antibody, immuno–colocalization, and co–transfection experiments. Results: The Western blot analysis revealed that LGN, a GPR– and tetratricopeptide (TPR)–motif containing protein, is present in the retina. Immunohistochemical analysis showed that within the retina, LGN is primarily localized in the inner segments and synaptic terminals of photoreceptor cells. The two major forms of LGN, LGN–long (∼75 kDa) and LGN–short (∼30 kDa), are membrane–bound and cytoplasmic, respectively. Following light–dependent translocation to the inner segments and other compartments of photoreceptor cells, Gtα co–localizes with LGN. The complex of LGN with Gtα can be immunoprecipitated from retinal homogenates. The distribution of LGN in photoreceptor cells depends on the presence of transducin. In the Gtα knockout mice, LGN is drawn from the inner segment more towards the outer nuclear layer. Gtα also changes subcellular localization of LGN in transfected COS7 cells. Conclusions: LGN is present in the inner segments of photoreceptor cells, where it appears to interact with Gtα. LGN may play essential roles in transducin–dependent and independent functions of photoreceptor cells.

Keywords: photoreceptors • signal transduction • protein structure/function