Peptide Hormone Signal Transduction via a G Protein-Linked Receptor

These animations have been prepared with the objective of serving as teaching tutorials to assist undergraduate students in conceptualizing the complex dynamics of physiological processes -- especially as they relate to insects. I am sharing these animations with my colleagues that wish to link to them, or refer them to their students for the purpose of illustrating course lecture topics.

As a courtesy, to help me evaluate the usefulness of the animations, if you use them as an educator in your teaching or as a student in your studies, please send me an e-mail at: llkeeley@tamu.edu, and let me know if they are helpful. I would also point out that these are works in progress, and will be updated occasionally with more, or remodeled, scenes to improve their usefulness for instruction

Instructions for playing the animation

The movie starts on opening and the title page takes several seconds to play.

The control panel embedded in the movie allows you to control any scene.

The MENU button allows you to view any scene without viewing the preceding scenes.

SUMMARY

The hormonal products of neuro-endocrine cells in the animal nervous system are usually proteins or peptides. Protein and peptide hormones can be large molecules and are generally hydrophilic in their chemical properties. The hydrophilic nature of the peptide hormones means they do not readily pass through the hydrophobic phospholipids that comprise the cell membrane. Hence, peptide hormones do not enter their target cells to convey their signal (message) to the cell. Peptide hormones interact with a cell-surface receptor protein (a G protein-linked receptor) which transduces the message to the cell via intracellular second messengers.

This animation demonstrates the sequence of biochemical events for a hypothetical peptide hormone that stimulates a depolarization in the cell membrane as its physiological response.

Scene 1: External Overview

The scene opens with a view of a target cell. The unique feature of a target cell is the presence of a receptor for the hormone.  In this case, since the peptide hormone cannot enter the target cell to convey its message, the hormone receptor is exposed on the surface of the target cell.

The peptide hormone is too small to have disulfide crosslinks, salt bridges or other intra-chain interactions that provide 3-dimensional configurations to large proteins. Rather, the peptide hormone is a short, flexible chain that has no particular tertiary configuration and only assumes a particular, active configuration as it interacts with the exposed active site of the receptor.

The peptide hormone locks onto the active site of the receptor and conveys its message to the receptor (figuratively illustrated here as a "glow" that transfers into the target cell). The receptor carries the message into the cell where it is transduced (def. transduce -- convert a message into another form) into intracellular second messengers via a G-protein. The second messengers then stimulate a protein kinase enzyme to produce a physiological response to the original hormonal message.

In the case of this illustration, the physiological response is opening of a cation channel in the cell's plasma membrane. Opening the channel allows cations to enter the cell and results in a temporary, localized depolarization (reversal) of the electrical charge on the cell membrane.

Note the time lag between the interaction of the hormone with the receptor and the opening of the cation channel. This time lag represents the sequence of intracellular transductional events occurring between the receptor, the G-protein, the second messengers and the protein kinase that result in the cation channel opening.

The scene ends with a fade-out, internal view of the hormone receptor and the cell membrane and showing the interior components that play essential roles in the intracellular events of signal transduction.

Scene 2: Hormone Transduction

The scene opens with a replay of the events that occur externally. The hormone receptor is a large protein that consists of seven, hydrophobic transmembrane-spanning regions. The free N- (amine) terminus of the receptor protein is external to the cell, and the free C- (carboxyl) terminus is internal.

The peptide hormone assumes its active configuration as it approaches the extracellular active site of the receptor, locks into the active site and transfers its information. In this animation, the information transfer is simulated as a movement of a "glow" along the transmembrane-spanning segments of the receptor. This is only a symbolic depiction and is not to be construed as the actual mechanism of information transfer. The information transfer probably results from a conformational change in the receptor's 3-dimensional structure.

The information transfer to the interior of the cell results in a mechanical interaction between the receptor and a G-protein. G-proteins are heterotrimeric (three unlike subunits) proteins consisting of an a-, b- and g-subunits. The receptor - G-protein interaction results in a transfer of a guanosine trisphosphate (GTP) for guanosine diphosphate (GDP) on the a-subunit. The GTP activates the a-subunit, which separates from the b- and g-subunits and interacts with an inactive phospholipase c within the plasma membrane. The interaction with phospholipase c converts the GTP to GDP. The energy released by hydrolysis of the terminal phosphate bond of GTP is transferred from the a-subunit to the inactive phospholipase for its conversion to the active enzyme. After its interaction with phospholipase c, the a-subunit rejoins with the b- and g-subunits to form the inactive G-protein.

The activated phospholipase c degrades membrane phospholipids as substrate to produce two second messengers: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP₃ is released into the cytoplasm and binds with an IP₃ receptor on the membrane of the endoplasmic reticulum. Binding IP₃ stimulates the release of calcium ions that are stored in the endoplasmic reticulum. The released calcium ions, together with diacylglycerol, activate an inactive protein kinase.

Scene 3: Hormone Action

The activated protein kinase transfers the terminal high-energy phosphate (~Pi) from ATP to the proteins that comprise a cation channel spanning the plasma membrane of the target cell. The cation channel opens and allows extracellular cations such as sodium ions to enter the cell. The movement of sodium ions into the cell causes a localized depolarization of the cell membrane. The depolarization spreads electrotonically across the cell membrane and can serve as a signal to be conveyed to other cells or perform an action within the target cell.

Other physiological actions by G-protein signal transduction and intracellular second messengers could involve activation of a protein kinase to phosphorylate and activate an inactive enzyme. The active enzyme may be part of a biosynthetic pathway and the pathway produces metabolic products essential to physiological processes.

An example of the latter is observed in insects with the activation of inactive glycogen phosphorylase in the fat body by the hypertrehalosemic hormone. The activated glycogen phosphorylase degrades carbohydrate stored as glycogen in the fat body cells. The glycolytic products of glycogen degradation serve as precursors for the synthesis of trehalose, the major insect blood sugar. Hence the physiological action of the hypertrehalosemic hormone is to increase the carbohydrate circulating in the hemolymph.  A comparable response is observed in vertebrate animals with the activation of liver phosphorylase for the elevation of blood glucose in response to the hormone glucagon.