Molecule-receptor binding controlled synthesis

Other receptors are soluble proteins that exist in the cell cytoplasm. When complexed with their signaling molecules or ligands, they migrate to the nucleus where the receptor ligand complex binds specifically to sites on the nuclear DNA and controls protein synthesis. 

The H2 receptor is a 359-amino-acid protein in humans. It has some features similar to the Hi protein (e.g., N-terminal glycosylation sites) and phosphorylation sites in the C-terminal. An aspartic acid residue in the third transmembrane loop appears to be critical to agonist and antagonist binding, and threonine/aspartate and tyrosine/aspartate couples in the fifth transmembrane domain appear to be important for interaction of the imidazole part of the histamine molecule. It is positively coupled via Gas to activate adenylyl cyclase for synthesis of cyclic adenosine monophosphate (cAMP) as a second messenger. In some systems, it is coupled through Gq proteins to stimulate phospholipase C. It appears in some cells that other processes, such as breakdown of phosphoinositides, control of intracellular calcium ion levels, and phospholipase A2 activity, can be regulated by other cAMP-independent pathways. 

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