Retrovirally delivered random cyclic Peptide libraries yield inhibitors of interleukin-4 signaling in human B cells.

Inteins are polypeptide sequences found in a small set of primarily bacterial proteins that promote the splicing of flanking pre-protein sequences to generate mature protein products. Inteins can be engineered in a "split and inverted" configuration such that the protein splicing product is a cyclic polypeptide consisting of the sequence linking two intein subdomains. We have engineered a split intein into a retroviral expression system to enable the intracellular delivery of a library of random cyclic peptides in human cells. Cyclization of peptides could be detected in cell lysates using mass spectrometry. A functional genetic screen to identify 5-amino acid-long cyclic peptides that block interleukin-4 mediated IgE class switching in B cells yielded 13 peptides that selectively inhibited germ line epsilon transcription. These results demonstrate the generation of cyclic peptide libraries in human cells and the power of functional screening to rapidly identify biologically active peptides.