that is inactive when cholesterol levels are high, and active when cholesterol levels are low. When cholesterol levels fall, SREBP is released from the nuclear membrane or endoplasmic reticulum, it then migrates to the nucleus and causes the transcription of the HMG-CoA reductase gene. The translation (creating the enzyme from the mRNA transcript) of HMG-CoA reductase is inhibited by derivatives of mevalonate and by dietary cholesterol. The degradation of HMG-CoA reductase is tightly controlled. The part of the enzyme that is bound to the endoplasmic reticulum senses signals, such as increased cholesterol levels, that lead to its degradation or proteolysis. When HMG-CoA reductase is phosphorylated, its activity decreases. This means cholesterol synthesis is reduced when cell energy (ATP) levels are low. Function Cholesterol is an important lipid involved in metabolism, cell function, and structure. It is a structural component of the cell membrane, such that it provides structure and regulates the fluidity of the phospholipid bilayer. Furthermore, cholesterol is a constituent in lipid rafts. These are congregations of proteins and lipids (including sphingolipids and cholesterol) that float within the cell membrane, and play a role in the regulation of membrane function. Lipid rafts are more ordered or rigid than the membrane bilayer surrounding them. Their involvement in regulation stems mostly from their association with proteins; upon binding substrates, some proteins have a higher affinity for attaching to lipid rafts. This brings them in close proximity with other proteins, allowing them to affect signaling pathways. Cholesterol specifically acts as a spacer and a glue for lipid rafts; absence of cholesterol leads to the dissociation of proteins. Given its prevalence in cell membranes, cholesterol is highly involved in certain transport processes. It may influence the function of ion channels and other membrane transporters. For example, cholesterol is necessary for the ligand binding activity of the serotonin receptor. In addition, it appears to be very important in exocytosis. Cholesterol modulates the properties of the membrane (such as membrane curvature), and may regulate the fusion of vesicles with the cell membrane. It may also facilitate the recruitment of complexes necessary for exocytosis. Given that neurons rely heavily on exocytosis for the transmission of impulses, cholesterol is a very important part of the nervous system. One particularly relevant pathway in which cholesterol takes place is the Hedgehog signaling pathway. This pathway is very important during embryonic development, and involved in deciding the fate of cells