Diphosphate Nucleotide Kinase Across The Cell Membrane Information Transfer,

Recently, a family of guanosine nucleotlde regulatory "proteins were found and proved to be a common transmembrane mediater of many receptors and their physiological responses. The family of guanosine nucleotlde regulatory proteins (G or N proteins) are consisted of at least 4 structurally and functionally different members; namely Gs and Gi, the stimulatory and inhibitory GTP-binding protein abundant in brain its function is not clear, but there is evidence suggests that it maybe involved in the regulation of polyphosphoinositide metabolism; and Gt (transducin) which is associated with cyclic GMP phosphodiesterase for transducton of photo signals in rod and cone out segments. Recent evidences also suggest a role of G protein in regulating activities of ion channels.All of the G proteins are heterotrimers, with virtually identical β and γ subunit, and distinctive a subunit. The a subunits, with molecular weight ranging from 39 to 52k (?)oltons bind GTP and serve as substractes for ADP-ribosylation by the exotoxins of Vibrio cholerae (in Gs and Gt) and Bordetella pertussis (in Gi, Go and Gt). All a subunits possess intrinsic guanosine triphophatase (GTPase) activity and the receptor-mediated stimulation of GTPase has been demonstrated by reconstitution of purified G proteins with receptor containing membrane. Such recostituted preparations yield hormone-stimulated activities that are similar to those noted in plasma membranes, suggesting that the GTPase activity of the G protein play an important role in the regulatory mechanisms. In the basal state, a receptor is unliganded and unassociate with G protein. The α subunit of G proteins is bound with GDP and the β γ subunit is associated. In the presence of certain agonist of receptor, the G-GDP is associated with the receptor-agonist complex and (?)DP is replaced by GTP; binding GTP cause the dissociation of the α and βγ subunits of G protein. The result is the formation of -GTP, the activator (or inhibitor) of appropriate effector. After the hydrolysis of the bound GTP by the intrinsic GTPase of a subunit, the G protein returns to the basal state and the α -GDP reassociates with β γ subunits. According to this general model, the binding and hydrolysis of GTP are crucial steps in the membrane signal transduction.Nucleoside diphosphokinase (NDPK, ATP nucleoslde diphosphate phosphotransforase, EC 2.7.4.6) is a family of...