阿拉伯芥與哺乳類動物之 G-protein 訊息系統相關蛋白質具有物種間交互作用

Abstract

Heterotrimeric G-protein, composed of Gα subunit and Gβγ dimer, is one of the most prevalent cellular signaling molecules and relays outside stimuli to mediate a variety of physiological responses. The G-protein signaling cascades are switched on via the GTP bound Gα subunit and then off due to the GTP hydrolysis that is catalyzed by the intrinsic GTPase of Gα subunit. Furthermore, this signaling pathway can be modulated by a group of proteins, regulator of G-protein signaling (RGS), which function as GTPase activating proteins (GAPs) to accelerate the GTPase activity of Gα proteins and then shorten the duration of signal transduction. Accordingly, RGS has been served as a negative regulator in G-protein signaling and its functional characteristic also makes RGS as a potential therapeutic target. In mammalian, diverse RGS proteins form a RGS superfamily and are widely distributed in various tissues. However, in Arabidopsis, there is only one RGS protein (AtRGS1), the first RGS found to contain N-terminal seven-transmembrane domain. This study was designed to compare the similarity between the RGS domain of AtRGS1 and mammalian RGS proteins via fluorescent assay. The GAP activity for RGS proteins were conducted by the fluorescent GTP analogue, BODIPY TR-GTP, and the results exhibited that plant AtRGS1 can accelerate the GTPase activity of both cognate plant Gα (AtGPA1) and mammalian Gαi1, but this phenomenon was not consistent with mammalian RGS4, which only had GAP activity toward mammalian Gαi1 but not plant AtGPA1. These results were further confirmed by monitoring the interaction between Lucifer yellow-modified RGS and Gα proteins in real time. Moreover, the affinity assay also reported plant AtRGS1 had the similar affinity toward plant AtGPA1 and mammalian Gαi1, which further supported the existence of inter-species interaction. Finally, according to above results and coupling with structural and sequence analysis, it can be concluded that the plant AtRGS1 is more similar to the mammalian R4 subfamily RGS; however, AtRGS1 and AtGPA1 still contain some plant-specific residues to cause the barrier for inter-species interaction, thus blocking the interaction between mammalian RGS proteins and plant AtGPA1.