Question

What is a heterotrimeric G?protein? What is the role of each subunit of the trimer. Explain how this protein is switched on by the ??adrenergic receptor. Why is a g-protein sometimes called a time switch?

Answer 1

The heterotrimeric G proteins are basically the membrane proteins that are involved in the transmembrane signalling in the nervous system. They couple the activation of different types of plasma receptor to a variety of intercellular processes. There are three subunits to the heteromeric diamer- alpha, beta and gamma. These three subunits are mainly concerned with targeting a particular type of G protein to a specific type of receptor. The alpha subunit has two domains that are identified. These two domains are concerned with GTP ase activity and GTP binding site.The betagamma subunits bind to K+ ion channels and activate them, also they bind and modulate the activity of PI-PLCbeta and some other classes of adenylyl cyclase. The betagamma subunit also bind to several protein kinases, such as phosducin.

G proteins are called time switch because of their capability to bind to the substrate GTP along with breakingdown GTP into GDP and iP. They actually undergo an intrinsic cycle between the active and inactive state in relationship to their substrate binding. When G protein binds GTP it is in ACTIVE state, and undergoes conformational change. At the same point, GTP is ultimately hydrolysed to GDP and Pi, and become INACTIVE.