Mg2+ interacts with the α subunits of guanine nucleotide-binding regulatory proteins (G proteins) in the presence of guanosine-5'-[γ-thio]triphosphate (GTPγS) to form a highly fluorescent complex from which nucleotide dissociates very slowly. The apparent K(d) for interaction of G(α)·GTPγS with Mg2+ is approximately 5 nM, similar to the K(m) for G protein GTPase activity. G(βγ) increases the rate of dissociation of GTPγS from G(α)·GTPγS or G(α)·GTPγS·Mg2+ at low concentrations of Mg2+. When the concentration of Mg2+ exceeds 1 mM, G(βγ) dissociates from G(βγ)·G(α)·GTPγS·Mg2+. Compared with the dramatic effect of Mg2+ on binding of GTPγS to G(α), the metal has relatively little effect on the binding of GDP. However, G(βγ) increases the affinity of G(α) for GDP by more than 100-fold. High concentrations of Mg2+ promote the dissociation of GDP from G(βγ)·G(α)·GDP, apparently without causing subunit dissociation. The steady-state rate of GTP hydrolysis is strictly correlated with the rate of dissociation of GDP from G(α) under all conditions examined. Thus, there are at least two sites for interaction of Mg2+ with G protein-nucleotide complexes. Furthermore, binding of G(βγ) and GTPγS to G(α) is negatively cooperative, while the binding interaction between G(βγ) and GDP is strongly positive.
|Original language||English (US)|
|Number of pages||5|
|Journal||Journal of Biological Chemistry|
|State||Published - Jan 1 1987|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology