Final answer:
Insigs is the cholesterol sensor that covers the ER exit signal on SREBP/SCAP, preventing SREBP activation and regulating cholesterol homeostasis in cells when cholesterol levels are adequate.
Step-by-step explanation:
The cholesterol sensor that covers the ER exit signal on SREBP/SCAP is Insigs. Insigs is crucial for regulating cholesterol synthesis in cells. When cholesterol levels are sufficient, Insigs binds to SREBP (Sterol Regulatory Element-Binding Protein) and SCAP (SREBP Cleavage-Activating Protein), retaining them in the endoplasmic reticulum (ER). This prevents SREBP from moving to the Golgi apparatus, where it would normally be processed and activated to upregulate the expression of genes involved in lipid biosynthesis. Therefore, by controlling the localization of SREBP/SCAP, Insigs serves as an effective mechanism to maintain cholesterol homeostasis in the cell.