In vivo production of cyclic peptides for inhibiting protein—protein interaction

A method comprising:

• a) providing a plurality of cells, each comprising recombinant nucleic acids encoding a first and a second interacting protein, wherein said first and second interacting proteins are:
• a ubiquitin activating agent and a ubiquitin conjugating agent,
• a ubiquitin conjugating agent and a ubiquitin ligating agent,
• or
• a ubiquitin ligating agent and a substrate of said ubiquitin ligating agent;
• b) introducing into said cells a library of fusion nucleic acids encoding intein-catalyzed cyclic peptides comprising from 5′ to 3′:
• i) nucleic acid encoding a C-terminal intein motif;
• ii) nucleic acid encoding a peptide;
• and
• iii) nucleic acid encoding an N-terminal intein motif;
• and
• c) identifying a cyclic peptide that inhibits binding of said interacting proteins.