Document Type

Honors Capstone Project

Date of Submission

Spring 5-1-2007

Capstone Advisor

Dr. Scott Erdman

Honors Reader

Dr. John Belote

Capstone Major


Capstone College

Arts and Science

Audio/Visual Component


Capstone Prize Winner


Won Capstone Funding


Honors Categories

Sciences and Engineering

Subject Categories

Biochemistry | Biochemistry, Biophysics, and Structural Biology


Cell Membranes are composed of several different lipid and sterol products. Among these are, chiefly, phospholipids, glycolipids, sphingolipids, various proteins and sterols. The sterol that is prevalent in fungi, including yeast, is ergosterol. It plays the same physiological role as cholesterol in mammalian cells. That is, mainly, to control membrane fluidity. Membranes in general are extremely important to the normal functioning of any cell and its sub-cellular compartments. The primary factor in the normal functioning of a membrane is the relative composition of the previously mentioned components. Even though there is a high amount of traffic between different membranes within a cell, each one requires its own distinct composition in order to function properly. How cells maintain these distinct compositions is of great interest because abnormal sterol levels have been linked to many diseases in humans, including heart disease and Alzheimer’s disease.

In a previous study, a yeast knockout library was screened for sensitivity to a class of anti-fungal drugs called triterpene glycosides. Triterpene glycosides, or TTG, are drugs that work by disturbing membranes. Of the yeast mutants that were found to be hyper-sensitive to TTG, two, ERG4 and ERG5, were found to be involved in the ergosterol biosynthesis pathway. Erg4p, called C-24(28) reductase, is the last enzyme in the ergosterol biosynthesis pathway. When this gene is knocked out, there is a complete lack of ergosterol in the membrane. Instead, the enzyme’s substrate, ergosta-5,7,22,24(28)-tetraen-3beta-ol, accumulates in the membrane. Likewise, when the ERG5 gene is deleted, the enzyme’s substrate, ergosta-5,7,24-trien-3beta-ol, accumulates in the place of ergosterol. The Erg5p is known as C-22 desaturase and immediately precedes C-24(28) reductase in the biosynthetic pathway. Another gene displaying the hyper-sensitive phenotype, OSH3, is involved in the transport of sterols to and from the plasma membrane, and the esterification of exogenous sterol products, though its exact function is yet uncharacterized.

We have begun high copy suppression screens, usingTTG, which seek to identify compensation mechanisms between the major components of membranes. Unfortunately, one of these screens did not give enough data to justify continuing the project. The other, however, has been successful, yielding several suppression candidates, and the first phase of the screen is drawing to a close. Now that suppressors have been found, the lab will then work to understand how these particular genes compensate for the complete lack of ergosterol in the yeast membranes and rescue the hypersensitive phenotype. These studies seek to lay the groundwork for understanding the interplay of the various membrane components, the importance of their relative composition in a membrane, and the process by which cells regulate the compositions of membrane particles, particularly the primary yeast sterol, ergosterol.

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