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Y-K Ho; The Origin of Allosteric Binding of Transducin to Rhodopsin . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1421.
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Purpose:Transducin activation by rhodopsin has been shown under positive allosteric control (Wessling-Resnick and Johnson, JBC 262,3996 (1987)). The origin of this allosteric behavior has not been identified. Chemical cross-linking study (Hingorani, et al., JBC,259,6694 (1984)) suggested that transducin forms supermolecular structure as large as [Tαßγ]4 in solution. In this study, the role of the tetrameric [Tαßγ]4 on the allosteric behavior of transducin is investigated. Methods:Correlations of the tetrameric transducin [Tαßγ]4 and its binding to rhodopsin have been studied by analytical ultracentrifugation and chemical modification methods. Specific inhibitory effect of bovine serum albumin (Buzdygon and Leibman, JBC,259,14567,(1984)) on the allosteric binding of transducin was examined. Results:Ultracentrifugation study confirmed that transducin in solution is under equilibrium of [Tαßγ]4 and Tαßγ. Upon activation with Gpp(NH)p, these complexes dissociate into subunit of Tα-Gpp(NH)p and Tßγ. Chemical modifications such as ADP-ribosylation with pertussis toxin, sulfhydryl modification with N-ethylmaleimide and S. aureus V8 protease treatment of transducin which inhibited transducin binding to rhodopsin also dissociated [Tαßγ]4 to Tαßγ. Direct binding of transducin to rhodopsin exhibited a Hill coefficient of 2.6. Bovine serum albumin (BSA) abolished the allosteric behavior. BSA also inhibited pertussis-toxin-catalyzed ADP-ribosylation of transducin indicating an interruption of the [Tαßγ]4 structure. Conclusion:The tetrameric [Tαßγ]4 is the major form of transducin that interacts with rhodopsin. The formaiton of the tetrameric transducin complexes fully explains the allosteric binding behavior of transducin to rhodopsin.
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