Molecular Mechanism of Rhodopsin Synthesis and Transport in Drosophila Phatoreceptor Cell.

为了阐明视紫红质在果蝇感光细胞中合成和转运的分子机制，本研究从以下几个方面进行了研究：（1）视蛋白分子构象的重要性;（2）果蝇Rab蛋白的鉴定;（3）视蛋白转运与信号转导系统的相互作用。本研究阐明：（1）当视蛋白的氨基酸取代或发色团饥饿抑制了视蛋白分子的正常形成时，糖基化视蛋白（视蛋白的未成熟形式）的加工受到抑制，并积累了含有大（高甘露糖）寡糖链的视蛋白。体外和体内实验均表明，翻译后立即在视蛋白的N-末端区域连接单一寡糖链。糖链对于稳定未成熟的视蛋白至关重要，但必须在形成功能性视紫红质之前进行处理。(2)类胡萝卜素剥夺影响果蝇Rab基因的转录。本研究在果蝇视网膜中鉴定了14种Rab蛋白。其中四个（包括一个新成员的Rab）显示类胡萝卜素依赖的mRNA表达。由于类胡萝卜素对Rab表达的调控作用在脑中也有类似的发现，因此这种调控作用可能广泛存在于神经系统中。(3)强光照射引起的视网膜变性是由于视蛋白合成受到抑制，微绒毛膜异常内陷到胞浆中所致。视蛋白合成的抑制是由于光转导系统过度激活，阻断了视蛋白从rER向微绒毛膜的转运。在此基础上，建立了视蛋白合成的粗略模型。

In order to elucidate the molecular mechanism of rhodopsin synthesis and transport in the Drosophila photoreceptor cells, investigations were carried out with special reference to the following points : (1) importance of molecular conformation of opsin, (2) identification of Drosophila Rab proteteins, and (3) interaction between opsin transport and signal transduction system. The present research then elucidated that : (1) when normal forlding of opsin molecule was prohibited by the amino acid substitution of opsin or chromophore starvation, processing of the glycosylated opsin (immature form of opsin) was inhibited, and opsin bearing large (highmannose) oligosaccharide chain was accumulated. Both in vitro and in vivo experiments indicated that single oligosaccharide chain was attached at the N-terminal region of opsin immediately after translation. The sugar chain was essential to stabilize the immature opsin, but must be processed before forming a functional rhodopsin. (2) Carotenoid deprivation affected the transcription of drosophila Rab genes. In this study, 14 kinds of Rab proteins were identified in the Drosophila retinal. Four of them (including one novel member of Rab) showed carotenoid-depandent mRNA expression. Since the similar regulation of Rab expression with carotenoid was also found in the brain, this regulation may run widely in the nervous system. (3) Retinal degeneration caused by the irradiation with strong light was resulted from the inhibition of opsin synthesis, as well as abnormal invagination of microvillar membrane into cytosol. The inhibition of opsin syntheisi was caused by blocking of opsin transport from rER to microvillar membrane, which was induced by the hyper-activation of phototransduction system. Based upon these results, a rough model for opsin synthesis was constructed.