Cholesterol biosynthesis regulation via the SREBP (sterol regulatory element binding protein) pathway has previously been elucidated to exhibit a switch-like response in the ER at ~ 5 mol% cholesterol. It has been proposed that this threshold value is mainly due to the control of the concentration at which cholesterol becomes accessible by the phospholipid composition of the ER membrane. We have conducted experiments to investigate this hypothesis. Perfringolysin O (PFO) was applied as a sterol sensor that binds membranes and oligomerizes upon cholesterol exposure in artificial membranes. Using this system, we detected a progressive enhancement of cholesterol accessibility as more PE (phosphatidylethanolamine) and PS (phosphatidylserine) phospholipids were incorporated into SUVs composed of PC (phosphatidylcholine). Further studies involving the uptake of various biologically relevant fatty intercalants (triglycerides, diglycerides, fatty alcohols) into PE/PS-containing vesicles reduced the threshold to a value as low as 2 mol% cholesterol. We discuss the hypothetical contributions of PE, PS, and various fatty intercalants in lowering the threshold concentration at which the SREBP pathway decides to transcriptionally activate or deactivate cholesterol biosynthesis.