Cell signalling G proteins

Cell signalling G proteins

Cell signalling is a type of communication that cells do with one another or with itself. It involves transmitting ligands which are signalling molecules which then bind to a receptor protein to create a change such a release a chemical or hormone.

G-Proteins guanine-nucleotide binding protein.

G-Proteins coupled receptor structure:

-7 transmembrane alpha helices with a extracellular N terminus and a cytosolic C terminus containing a ligland (signalling molecules) binding site also there is a cytosolic loop between segment 5 and 6 binds specific G proteins.

GPCR (G protein coupled receptors) are very common types of receptors on the plasma membrane and most drugs produced work around these types of receptors. 

1-Many G protein coupled receptors have a large extracellular ligland binding domain when an appropriate  protein ligland binds to this domain the receptor undergoes a conformation change that is transmitted to its cytosolic regions which now activate a trimeric GTP binding protein also called g protein.

2-A G protein consists of 3 protein sub units; Alpha beta and Gamma. Alpha and Gamma have contently attached lipid tails which help anchor the G protein to the membrane, In the absence of a signal the alpha sub unit has a GDP bound to it and the G protein is inactive .

3-An activated receptor induces a conformational change in  the alpha subunit causing the GDP to dissociate and GTP in the cytsol binds in its place which  causes a conformational change activating both alpha sub unit and beta gamma complex now it can regulate the activity of target proteins in the plasma membrane

4-The activated target proteins then relay the signal to other components in the signalling cascade eventually the alpha sub unit hydrolyses its bound GTP  to GDP which inactivates the sub unit. This is sped up by a protein called RGS (regulator of g protein signalling). The inactivated alpha sub unit reforms with the beta and gamma sub units which turns off other downstream events.

5-As long as the signalling receptor remains stimulated it can continue to activate G proteins but after a very long time it begins to get inactive. so a receptor kinase phosphorylates the cytosolic portions of the activated receptor and then binds to a high affinity to arrestin protein which inactivates the receptor by preventing any interactions with G proteins.

**REMEMBER TO STAY POSITIVE LIKE A PROTON!!**