Relationship between intracellular sterol content and sterol esterification and hydrolysis in Saccharomyces cerevisiae.

The relationship between the supply of free sterol and the synthesis of steryl esters by an auxotroph of Saccharomyces cerevisiae has been examined in order to understand the role of cellular free sterol content in the regulatory interactions of sterol esterification. Our results show that the yeast cells must maintain an essential, low level of free sterol that is critical for growth. An additional, expandable pool of free sterol is maintained by the cells, provided there is adequate available sterol. As the quantity of sterol in the expandable pool increases, there is a progressively increasing rate of sterol esterification, which is consistent with the results from in vitro assays of acyl-Co A:ergosterol acyltransferase. Some increases in acyltransferase activity were insensitive to the protein synthetic inhibitor, cycloheximide. The data support the conclusion that sterol interconversion between the free and esterified forms is directed toward maintaining the essential amount of free sterol and that the activity of the relevant sterol enzymes in this organism are modulated in response to intracellular sterol content.