How G protein help in signal transmission in to the cell from an incoming ligand & outline the steps.

• Ligand bind G protein coupled receptor which has α ,β & γ subunits.
• After binding, GTP in α dissociates from β & γ( all 3 subunits can produce physiological effects)
• Causes production of a 2^nd messenger eg:-cAMP via activation of adenylyl cyclase.
• cAMP produce activated protein kinase A( pK_A)  à  pK_A phosphorylate certain molecules & activate /inhibit  à brings about the physiological action
• When work is done , intrinsic GTPase activity of GTP-α converts GTP to GDP   à α ,β & γ subunits reassociates  à action terminates.