G protein alpha subunit

 A G-protein is used in cell signalling and has 3 subunits including the alpha subunit. The protein is bound to the phospholipid bilayer to interact with a G-protein coupled receptor when a signal is present. The alpha subunit can bind to either GDP or GTP to act as a switch for when the protein is active or inactive. When inactive, the alpha subunit binds to GDP until a signal molecule reaches the G-protein coupled receptor causing a conformational change to occur which then activates the G-protein. This activation causes the alpha subunit to act like a guanine nucleotide exchange factor by releasing GDP and binding to GTP and then the subunits dissociate from one another. When active the alpha subunit can relay messages into the cell to activate proteins and enzymes and then hydrolyses the bound GTP to GDP to become inactive again.

References:

• Alberts, B. et al. (2008) Molecular Biology of the Cell. 5th Edition. New York: Garland Science.
• Nature. Essentials of Cell Biology, Unit 4: How Do Cells Sense Their Environment?. Available at: http://www.nature.com/scitable/ebooks/essentials-of-cell-biology-14749010/122997540 [Accessed 25.11.14]