Textual description of G protein animations

The two cell biology animations described here are:

The Trimeric G Protein Cycle
Homologous Desensitization and Receptor Endocytosis.

In both of these animations, a horizontal bar, which represents the cell membrane, stretches across the screen. The area above the bar represents space outside the cell, and the area below the bar respresents the area inside the cell. The receptors, proteins, enzymes, and other things outside the cell, inside the cell, or inside the cell membrane, are all represented by different geometric shapes, some of which are labeled with letters to distinguish them from similar parts. Activated parts are represented by shades of green, and inactive parts are represented by shades of red.

These are the steps shown in the animation of the trimeric G protein cycle:

When the ligand binds the receptor, the receptor is activated.
When the alpha subunit exchanges GDP for GTP, the G protein becomes active.
The alpha subunit interacts with the target effector.
The activated target effector increases the amount of a second messenger.
The second messenger interacts with a protein kinase.
The protein kinase becomes activated.
The activated protein kinase phosphorylates downstream target proteins. Some target proteins are activated, some are inactivated, and some are degraded.
When the ligand dissociates from the receptor, the receptor becomes inactive.
When the alpha subunit dissociates from the target effector, the target effector becomes inactive.
The alpha subunit hydrolyzes GTP, forming GDP and releasing Pi.
The alpha and beta/gamma subunits reassociate and become inactive.
A second messengerase decreases the second messenger concentration.
The second messenger dissociates from the protein kinase, which becomes inactive.
The target proteins are dephosphorylated by a phosphatase.

These are the steps shown in the animation of homologous desensitization and receptor endocytosis:

When the ligand binds the receptor, the receptor is activated.
When the alpha subunit exchanges GDP for GTP, the G protein becomes active.
The beta/gamma subunit activates the G protein receptor kinase (GRK).
The G protein receptor kinase phosphorylates the receptor.
The phosphorylated receptor has decreased affinity for the ligand.
Adaptor protein is recruited to the receptor.
Clathrin is also recruited to the receptor.
Clathrin surrounds receptors on the cytosolic side of the membrane, with adaptor proteins between the receptors and the clathrin.
the membrane and receptors invaginate, forming an indent into the cytosol.
The clathrin-coated membrane begins to form a coated vesicle.
The clathrin-coated membrane continues to form a coated vesicle.
The clathrin-coated vesicle is completely formed (and detached from the membrane).
The vesicle uncoats.
The uncoated vesicle fuses with an intracellular membrane component, the endosome.
A phosphatase removes the phosphates, activating the receptors.
The receptors leave the endosome in a new membrane vesicle.
The new membrane vesicle returns to the plasma membrane.

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