We have expressed libraries of peptides in mammalian cells to select for trans-dominant effects on intracellular signaling systems. As an example-and to reveal pharmacologically relevant points in pathways that lead to Taxol resistance-we selected for peptide motifs that confer resistance to Taxol-induced cell death. Of several peptides selected, one, termed RGP8.5, was linked to upregulation of expression of the gene ABCB1 (also known as MDR1, for multiple drug resistance) in HeLa cells. Our data indicate that trans-dominant effector peptides can point to potential mechanisms by which signaling systems operate. Such tools may be useful in functional genomic analysis of signaling pathways in mammalian disease processes.

Fas is a cell surface receptor that can transmit signals for programmed cell death. Using a retroviral expression system, we have demonstrated that a short peptide derived from the cleavage site in a cellular target of a pro-apoptotic cysteine protease can be expressed within intact cells with sufficient activity to inhibit Fas-mediated apoptosis. In vitro analysis demonstrates that this retrovirally-expressed peptide is as potent as 150uM levels of the chemically synthesized peptide. Furthermore, using retroviral peptide library-based functional cloning we identified variants of this peptide with apparent anti-apoptotic activity. This approach is likely to lead to the identification of peptide variants with activity against a variety of signaling processes, both normal and pathological.