Mutations in the
rhodopsin gene cause approximately one-tenth of
retinitis pigmentosa cases worldwide, and most result in endoplasmic reticulum retention and apoptosis. Other
rhodopsin mutations cause receptor mislocalization, diminished/constitutive activity, or faulty
protein-
protein interactions. The purpose of this study was to test for mechanisms by which the autosomal dominant
rhodopsin mutation Ter349Glu causes an early, rapid
retinal degeneration in patients. The mutation adds an additional 51
amino acids to the C terminus of the
protein. Folding and
ligand interaction of Ter349Glu
rhodopsin were tested by ultraviolet-visible (UV-visible) spectrophotometry. The ability of the mutant to initiate phototransduction was tested using a radioactive filter binding assay. Photoreceptor localization was assessed both in vitro and in vivo utilizing fluorescent immunochemistry on transfected cells, transgenic Xenopus laevis, and knock-in mice. Photoreceptor ultrastructure was observed by transmission electron microscopy. Spectrally, Ter349Glu
rhodopsin behaves similarly to wild-type
rhodopsin, absorbing maximally at 500 nm. The
mutant protein also displays in vitro
G protein activation similar to that of WT. In cultured cells, mislocalization was observed at high expression levels whereas ciliary localization occurred at low expression levels. Similarly, transgenic X. laevis expressing Ter349Glu
rhodopsin exhibited partial mislocalization. Analysis of the Ter349Glu
rhodopsin knock-in mouse showed a rapid, early onset degeneration in homozygotes with a loss of proper rod outer segment development and improper disc formation. Together, the data show that both mislocalization and rod outer segment morphogenesis are likely associated with the human phenotype.