Guanine nucleotides and magnesium dependence of the association states of the subunits of transducin

Abstract
When GTPγS is bound to transducin (T), the two subunits Tα·GTPγS and Tβγ dissociate, independently of the ionic environment. When GDP is bound, these subunits are associated as a monomeric Tα·GDP‐Tβγ complex of 75 kDa when the ionic environment is comparable to that of the cytoplasm, but they dissociate in the presence of 10–100 mM Mg2+ or Ca2+ Using this property, the subunits could be separated and purified by a rapid one‐step procedure on an ion‐exchange column (FPLC), and their molecular masses were verified by neutron small angle scattering. The physiological relevances of the dissociating effect of Mg2+ are discussed.