在嗜鹽古細菌 Haloarcula marismortui 發現一種新型態的感受型光感蛋白質

Abstract

光感受器 (photoreceptors) 在微生物體內對於感受週遭環境刺激，例如：光，扮演重要的角色，對於微生物生存是必需的。在已知的微生物光感受器中可分為兩類，一為離子幫浦，另一為感受型視紫蛋白質。在 Halobacteria salinarum 中 bacteriorhodopsin (BR) 為離子幫浦的其中一種，負責將細胞內的質子打到細胞外產生質子梯度，進而促使能量的產生。感受型視紫蛋白質主要感受週遭不同波長的光而傳遞驅離或吸引訊號。sensory rhodopsin I (SR I) 和其相對應的觸發器，HtrI，具有調控正趨光及負趨光性的功能。sensory rhodopsin II (SR II) 和其相對應的觸發器，HtrII，只有調控負趨光性的功能。 根據 2004 年完成的 H. marismortui 的基因體計畫，在其基因註解 (annotation) 中，有六個光感受器，是目前所有知道的單一嗜鹽古細菌中最多的。經由表現和特徵吸收峰的測定發現，這六個光感受器分別為三個離子幫浦，兩個感受型視紫蛋白質和一個功能未知的視紫蛋白質。本論文主要著重於這個功能未知，為本實驗室命名但尚未深入研究之光感受器sensory rhodopsin modulator ﹙SRM﹚，及其觸發器，HtrM。 當照射到不同波長的光時，SRM 可藉由本身構形上變化使其在基態和中間激發態﹙即 M state﹚做轉換。利用生物資訊軟體進行分析HtrM，發現它是具有兩個穿膜螺旋區域和胞內出膜延伸之C端結構的細胞質部分，同時它在羧端具有一個 Hamp domain﹙Histidine kinase, Adenylyl cyclases, Methyl-accepting chemotaxis proteins, and Phosphatases domain﹚，這是已知可以和其他蛋白質產生高度交互作用之蛋白質功能區塊。本研究提出SRM之功能假說，認為這個未知功能的蛋白質，是藉由改變基態和激發態之間的比例來調控其光感訊息。

Photoreceptors play important roles in responding to environmental stimuli like light and they are essential for surviving in microbes. Ion pumps and photosensory are two main types of microbial rhodopsin among known microbial photoreceptors. Proton pump, like bacteriorhodopsin 1 in Halobacteria salinarum, pumps out proton to create a proton gradient for eventually energy generation, while photosensory rhodopsins, on the other hand, mediate repellent or attract signal in response to different wavelength of light. Sensory rhodopsin I (SR I) mediates both attract/repellent responses together with its cognate partner protein, HtrI, while sensory rhodopsin II (SRII) is solely responsible for repellent response when working together with its cognate transducer, HtrII. The genome project for Haloarcula marismortui predicted the existence of six photoreceptors, a most abundance and variety among any single archaeon. Overexpression and absorbance measurements of those six photoreceptors concluded them as three ion pumps, two sensory rhodopsins and one unknown type rhodopsin. Here, we propose that this function-unknown protein as a new type of modulatory photoreceptor based on the results that this SRM can switch between ground state and M-intermediate state when exposed to different light wavelength. Also, we cloned and identified the SRM cognate partner transducer, HtrM, which has two trans-membrane region followed by a very short cytoplasmic region containing a HAMP domain in the C-terminus. Finally, we propose SRM modulates photosensory signal for H. marismortui by changing the ratio of Ground/M-intermediate state in response to different wavelength of light.