Allosteric behavior in transducin activation mediated by rhodopsin. Initial rate analysis of guanine nucleotide exchange

Wessling-Resnick, M.; Johnson, G.L.

Journal of Biological Chemistry 262(8): 3697-3705


ISSN/ISBN: 0021-9258
PMID: 3102494
Accession: 004699467

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Photolyzed rhodopsin acts in a catalytic manner to mediate the exchange of GTP for GDP bound to transducin. We have analyzed the steady-state kinetics of this activation process in order to determine the molecular mechanism of interactions between rhodopsin, transducin, and guanine nucleotides. Initial velocities (V0) of the exchange reaction catalyzed by rhodopsin were measured for various transducin concentrations at several fixed levels of the GTP analog, -1 show no evidence of allosterism and are a parallel series. Furthermore, the allosteric behavior observed in the activation of transducin is also witnessed in the rhodopsin-catalyzed guanine nucleotide exchange of the G protein's purified .alpha. subunit in the absence of the .beta.cntdot.gamma. subunit complex. The latter observation implies that the molecular basis for allosterism in the activation process residues in the interactions between the photoreceptor and transducin's .alpha. subunit.