Transduction Systems:

• Function to translate information contained in hormonal messages, when these are sensed by a receptor, into a language that can be interpreted and acted upon by target cells.

• For proteins, peptides, and hormones with a significant ionic charge at neutral pH, receptors are usually integral membrane proteins located at the cell surface.  When hormones bind to the receptors, the receptors interact with membrane-bound or intracellular transducer proteins to begin the cascade of events leading to cellular response.

• Some membrane receptors, e.g., the acetyl-choline receptor, act as ion channels that open or close in response to hormone binding and induce changes via changes of the intracellular ion/charge balance.

• For many lipophilic hormones, e.g., steroids or thyronines, receptors are intracellular, usually intranuclear, proteins.  When their specific ligands bind, the hormone-receptor complexes undergo conformational changes that allow them to interact with specific hormone recognition sites (HREs) in the DNA of the regulatory regions of certain genes.

• These transduction processes involve allosteric changes in receptor and/or transducer protein shape.  The signaling cascades of membrane-bound receptors almost universally involve protein phosphorylation by kinases.  These kinases may be activated initially via  generation of secondary messengers produced by allosteric activation of enzymes like adenylyl or guanylyl cyclase or via unmasking of kinase activities that are part of the cytoplasmic portions of the receptor proteins themselves.

• Both allosteric changes and/or phosphorylation, which often changes protein charge, alter protein shape and/or intercellular location and protein function.  These are exactly the kind of changes that would be needed to trigger a biochemical and cellular response.  This may occur without the intervention of protein synthesis and therefore may be very rapid, milliseconds to minutes.

• Opening or closing of ion channels will also precipitate rapid responses either directly or via the intervention of phosphorylation cascades with their associated changes in protein functions.

• Intranuclear receptor transduction also involves allosteric and phosphorylation changes.  It normally triggers changes in gene transcription and subsequent protein production and frequently modulates changes during a longer time course of minutes to days.