[Effects of chloride ion on the activation of G proteins]

G proteins act as signal transducers that couple receptors to effectors through membranes. G proteins are in a GDP-bound form in the basal state, which can interact with receptors. The hormone-bound receptor promotes GDP-GTP exchange of G proteins. The GTP, Mg(2+)-bound G proteins can activate effector molecules, which in turn produce second messengers. The GTP, Mg(2+)-form is converted to the GDP-form by the GTP-hydrolysis activity of G protein alpha-subunits. Cl- ions modulate the GDP-GTP cycle of G proteins. Cl- ions, in the presence of Mg2+, decelerate the receptor-independent spontaneous release of GDP (ca. 6-fold by 100 mM NaCl), which will result in a lowering of the basal level of second messengers. On the other hand, the GTP-hydrolysis activity of G proteins was suppressed by 3-fold with 100 mM NaCl, which will result in keeping the G proteins in an active form for a longer time. In the absence of Mg2+, Cl- ion mimics Mg2+ to convert GTP-bound G proteins to an active form (dissociated form), although the effects are weaker than those of Mg2+. These effects are very different from F-, which is another halogen ion that interacts with G proteins.