Higashide T, McLaren M J, Inana G
Bascom Palmer Eye Institute, University of Miami School of Medicine, Florida 33136, USA.
Invest Ophthalmol Vis Sci. 1998 Apr;39(5):690-8.
To characterize further HRG4, a novel photoreceptor protein recently identified by subtractive cDNA cloning, by sequence analysis and immunolocalization.
The rat homolog of HRG4, RRG4 was expressed and used to prepare an antibody. The antibody was used in Western blot analysis, and immunofluorescent localization at the light and electron microscopic levels of HRG4-RRG4 protein. The HRG4-RRG4 sequence was also analyzed for homologies.
HRG4-RRG4 showed 57% homology with unc-119, a Caenorhabditis elegans neuroprotein causing defects in locomotion, feeding, and chemosensation when mutated. By Western blot analysis, the HRG4-RRG4 protein was demonstrable only in retina and was soluble in nature. Immunofluorescence microscopic study of human and rat retinas, using the HRG4-RRG4 antibody, and other rod and cone photoreceptor-specific antibodies showed that the HRG4-RRG4 protein is localized in the outer plexiform layer of the retina in the synaptic termini of rod and cone photoreceptors. Electron microscopic immunolocalization showed the protein in the cytoplasm and on the presynaptic membranes of the photoreceptor synapses.
The homology to unc-119 and localization to the photoreceptor synapse are suggestive of a function for HRG4-RRG4 in photoreceptor neurotransmission. HRG4 is the first photoreceptor-enriched synaptic protein to be reported, suggesting that its function may be unique to the specialized ribbon synapses formed between photoreceptors and the horizontal and bipolar cells of the retina.
通过序列分析和免疫定位进一步鉴定HRG4，这是一种最近通过消减cDNA克隆鉴定出的新型光感受器蛋白。
表达HRG4的大鼠同源物RRG4，并用于制备抗体。该抗体用于蛋白质免疫印迹分析，以及在光镜和电镜水平对HRG4-RRG4蛋白进行免疫荧光定位。还分析了HRG4-RRG4序列的同源性。
HRG4-RRG4与unc-119显示出57%的同源性，unc-119是一种秀丽隐杆线虫神经蛋白，突变时会导致运动、进食和化学感觉缺陷。通过蛋白质免疫印迹分析，HRG4-RRG4蛋白仅在视网膜中可检测到，并且本质上是可溶的。使用HRG4-RRG4抗体以及其他视杆和视锥光感受器特异性抗体对人和大鼠视网膜进行免疫荧光显微镜研究表明，HRG4-RRG4蛋白定位于视网膜外网状层中视杆和视锥光感受器的突触末端。电镜免疫定位显示该蛋白存在于光感受器突触的细胞质和突触前膜上。
与unc-119的同源性以及定位于光感受器突触提示HRG4-RRG4在光感受器神经传递中具有功能。HRG4是首个被报道的富含光感受器的突触蛋白，表明其功能可能对于光感受器与视网膜水平细胞和双极细胞之间形成的特殊带状突触而言是独特的。
