Interaction of nucleoside diphosphate kinases with rhodopsin/G-protein transducin complex in the bleached bovine retinal rod outer segment membranes. Possible mechanism of extremely rapid transducin activation

Orlov, N.Y.; Freidin, A.A.; Orlov, D.N.; Kimura, N.

Biologicheskie Membrany (Moscow) 20(1): 45-51


ISSN/ISBN: 0233-4755
Accession: 010862502

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It is widely accepted that retinal rod outer segment (ROS) G-protein transducin (Gt) is activated by bleached light receptor rhodopsin (R*)-stimulated GDP/GTP exchange. Nevertheless, there are uncertainties as to whether the exchange is-rapid enough to provide the extremely high rate of Gt activation in living rods. In this work the modern data were summarized to analyze the another possible mechanism of G-protein activation that implies the receptor-stimulated phosphorylation of GDP in active site of G-proteins and assumes participation of nucleoside diphosphate (NDP) kinase. (Kimura, 1993; Weiland & Jakobs, 1989; Otero, 1990). The suggested mechanism of rapid Gt activation is based on the following data: (1) the phenomenon of high-affinity GTP(S)-dependent interaction of ROS NDP kinase and alpha isoform of rat recombinant NDP kinase with the R*-Gt complex in photoreceptor membranes (Orlov et al., 1996); (2) the existence of low-affinity interaction between NDP kinases and R* (Orlov et al., 1996; Orlov and Kimura, 1998); (3) the ability of NDP kinase to phosphorylate GDP covalently attached in active site of GTP-binding protein Rad (Zhu et al.,1999), and (4) well-known scheme of Gt activation (Stryer, 1986). According to the hypothesis, NDP kinase interacts with the R*-Gt complex and phosphorylates bound GDP. Then, active Gt-GTP releases, whereas low-affinity R*-NDP kinase complex has no time to dissociate before the rapid (mchlt1 ms) association with new Gt-GDP molecule. It means that R*-NDP kinase complex once formed will interact catalytically with many Gt-GDP molecules. It should be emphasized that the catalytic activity of NDP kinase is sufficiently high to accomplish rapid Gt activation cycle.