Effect of Intracellular Sterol Concentration on Sterol Esterification in Yeast

Abstract

Ergosterol (figure 1) was first isolated in 1889¹, and has been shown to be produced by 558 yeast cultures covering 60 species in 20 genera². Yet, very little is known about the precise physiological role for this compound. Activities as a steroidal precursor, in the maintenance of optimal membrane fluidity, and as a modulator of enzymic activity have all been speculated upon as the principal function for sterols in yeast. Supporting experimental results have been obtained for each of these activities. It occurred to us that rather than a single function, sterols may in fact have multiple roles in the general economy of the cell. Such a conclusion was based on the results from many experiments which failed to provide a reasonable unifying model for sterol physiology. We elected to investigate the possibility of multiple roles for sterols in yeast, formulating our experiments on three assumptions. Firstly, if there were indeed multiple roles, it seemed highly unlikely that each of the functions would have the same quantitative requirement for sterol. Secondly, it would also be unlikely that the disparate roles would have identical structural affinities for different features of the sterol molecule. Lastly, it seemed most reasonable that ergosterol would be able to satisfy all of the sterolic qualitative and quantitative requirements for the various functions.