Anchored Periplasmic Expression (APEx): A Powerful Technology for Antibody Discovery and Combinatorial Protein Library Screening

Life Sciences : Research Tools

Available for licensing


  • George Georgiou, Ph.D. , Chemical Engineering
  • Brent Iverson, Ph.D. , Chemistry and Biochemistry
  • Ki Jun Jeong, Ph.D. , Institute for Cellular and Molecular Biology
  • Barrett Harvey, Ph.D. , Chemical Engineering
  • Gang Chen, Ph.D. , Chemical Engineering
  • Jeffrey Thomas, Ph.D. , Chemical Engineering
  • Patrick Daugherty, Ph.D. , Chemical Engineering

Invention Description

APEx is a validated method to isolate binding polypeptides, including antibodies or antibody fragments, that recognize specific molecular targets. A library of polypeptide (e.g., antibody) mutants is constructed and expressed in E. coli bacteria. The mutant polypetides are expressed as fusion proteins that are anchored on the cytoplasmic (inner) membrane of the bacterium facing the periplasm. Subsequently, the outer membrane of the bacterium is made permeable by chemical treatments or other methods. Permeabilization of the bacterial outer membrane renders the polypeptides anchored on the membrane accessible to target molecules labeled with fluorescent dyes that have been added to the external solution. The bacterial clones expressing polypeptides that recognize the target bind to the fluorescently labeled target and in turn become fluorescent. The fluorescent bacteria expressing the desired proteins are enriched from the population by fluorescence activated cell sorting (FACS).


  • Antibody discovery and affinity maturation
  • Identifying protein:protein interactions
  • Protein expression maturation
  • Enzyme discovery
  • Discovery of cell and/or organ targeting peptides
  • Alternative to phage display


  • Quantitative results
  • Bacteria system and FACS are technically facile

Market potential/applications

APEx has been developed, externally validated, and licensed nonexclusively to several pharmaceutical companies. The technology has been used to engineer therapeutic antibodies that bind to anthrax toxin, high-affinity diagnostic antibodies for a drug of abuse, and other products.

Development Stage

Commercial product

IP Status