Chemical Approaches for Exploring Protein Prenylation in Living Cells

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PROJECT SUMMARY Protein prenylation is characterized by the addition of farnesyl (C15) or geranylgeranyl (C20) isoprenoids to cysteine residues located near the C-termini of different proteins. Although originally considered to be a rare modification, it is now clear that protein prenylation is widespread in eucaryotes and is of critical importance for a variety of proteins involved in oncogenesis, secretion, nuclear structure, and signal transduction. It has been estimated that as many as 2% of all proteins in mammalian cells are isoprenylated. This prevalence, coupled with the central role that many of these modified proteins play in cellular signaling, underscores the significance of this post-translational modification. Prenylation inhibitors were initially developed as therapeutic agents for cancer treatment. With the development of precision medicine approaches that target specific Ras- driven cancers, several clinical trials are ongoing. These inhibitors are also being investigated for the treatment of a wide variety of other disease including malaria, viral infections, Parkinson's disease and progeria. Two closely linked critical questions in the field of prenylation research concern what proteins are prenylated and how do they change in disease? What are clearly necessary are global methods that can compare the prenylomes in normal and disease states and allow those prenylated proteins whose levels change to be identified. If this were possible, it would reveal new targets for therapeutic intervention in these debilitating diseases. To address this, new isoprenoid probes for improved metabolic labeling will be synthesized and methods to decrease sample complexity and improve probe delivery will be developed. These will be utilized in quantitative proteomic experiments in cell culture and mouse models for disease. Another critical set of questions is what controls prenylation efficiency and is how is prenylation regulated? To address these questions, what are needed are strategies that allow the process of protein prenylation to be assayed in real time in live cells. This would allow prenylation reactions to be studied in a holistic manner in the presence of all relevant cellular components. It could open also up additional avenues for therapeutic intervention since it could reveal new regulatory interactions that could be targeted. To accomplish this, cell penetrating peptides with caged cysteine residues that mask their site of prenylation will be prepared and their subsequent prenylation monitored via microscopy after uncaging in a temporally controlled manner. Complementary experiments with full-length proteins, prepared by sortase ligation, containing the same light activated trigger will be used to explore potential interactions with chaperone proteins that may not be observed with the simpler peptide-based models.