Sterol Binding in ORPs (2012)

Undergraduate: Max Wolpert

Faculty Advisor: Vytas Bankaitis
Department: Chemistry

---

Sterols are important lipids that serve as essential membrane and signaling molecules in eukaryotic cells. The directed transport and signaling effects of these molecules are often accomplished via sterol-binding proteins. Two yeast proteins, Osh4 and Osh5 in the Oxysterol Binding Protein Related Proteins (ORPs) family, have been shown to bind sterol, and have close homology to their mammalian counterparts. Osh4 has been the focus of investigation for its important role in lipid metabolism pathways, but relatively little is known about Osh5. Osh5 is 66% identical and 77% similar to Osh4 in amino acid sequence, yet the two proteins have no functional overlap. However, based on its relationship to Osh4, Osh5 should have a similar interaction with sterol. In this investigation, we demonstrated by a sterol-binding assay that Osh5 binds sterol with similar affinity to Osh4 and that the Y97F Osh4 and Osh5 point mutants both have compromised sterol binding. This indicates that Osh5 binds sterol in a similar fashion to Osh4, and opens further questions about their differences in cellular function. To further investigate the sterol binding properties of Osh5 it is necessary to obtain a crystal structure, which is where current efforts are directed.