Peptides derived from bacterial membrane-targeting sequences can penetrate human cells and escape endosomal entrapment — solving a critical bottleneck in intracellular drug delivery, with D-amino acid versions showing the best endosomal escape.
2 of 6 MTS peptides escaped endosomesWhile all six bacterial membrane-targeting peptides entered cells, the D-amino acid versions d-EcMTS and d-TpMTS achieved the critical step of endosomal escape — a capability most cell-penetrating peptides lack
What the researchers found
Six peptides based on bacterial membrane-targeting sequences (MTSs) all showed cell-penetrating ability. Two D-peptide versions — d-EcMTS (from E. coli) and d-TpMTS (from T. pallidum) — demonstrated the additional critical ability to escape from endosomes after cellular uptake and localize at the endoplasmic reticulum (ER).
The utility was validated by successful intracellular delivery of green fluorescent protein (GFP), a large biomacromolecule that cannot enter cells on its own. The D-amino acid configuration provides protease resistance, addressing the stability limitation of natural L-peptide CPPs. The results suggest that the vast pool of bacterial MTSs represents an untapped resource for developing novel endosome-escaping cell-penetrating peptides.
Why it matters
Endosomal entrapment is the single biggest bottleneck in intracellular delivery of biological therapeutics — an estimated 99% of endocytosed molecules are degraded in lysosomes before they can act. Current CPPs get cargo into cells but mostly fail at endosomal escape. By mining bacterial membrane-targeting sequences — peptides that evolved to interact with and disrupt cellular membranes — this study identifies a new source of peptide delivery vehicles that naturally possess endosome-disrupting activity, potentially solving the efficiency problem that has limited biologics delivery for decades.
How the study worked
Six peptides based on bacterial membrane-targeting sequences were synthesized in both L- and D-amino acid configurations. Cell penetration was assessed using fluorescence microscopy and flow cytometry. Endosomal escape was evaluated by subcellular co-localization studies with endosomal and ER markers. Functional utility was demonstrated by intracellular delivery of GFP as a model cargo protein.
What this study cannot tell us
The study is entirely in vitro using cultured cells — in vivo delivery, biodistribution, and potential toxicity were not assessed. Only GFP was tested as cargo, and larger or differently structured cargoes may not be equally deliverable. The endosomal escape efficiency was demonstrated qualitatively but not quantified. The mechanism by which MTSs disrupt endosomal membranes was not elucidated in detail. Potential cytotoxicity from membrane-disrupting peptides at higher concentrations was not thoroughly explored.
How to read the evidence
This is a proof-of-concept in vitro study demonstrating a novel CPP design strategy. The cell biology experiments are standard for the field, but the absence of in vivo data and quantitative endosomal escape measurements limits the translational evidence.
When this study was published
Published in 2023, this is a recent study introducing a novel approach to CPP discovery by mining bacterial genome databases for membrane-targeting sequences.
The bigger picture
The cell-penetrating peptide field has been searching for efficient endosome-escaping sequences for over 20 years. Most current approaches — including TAT, penetratin, and other established CPPs — are limited by low endosomal escape efficiency. This study introduces a new design paradigm: instead of engineering escape ability into existing CPPs, start with peptides that naturally disrupt membranes (bacterial MTSs) and repurpose them for human cell delivery. The concept that bacterial evolution has already optimized membrane-disrupting peptides for a different purpose but applicable to drug delivery represents a creative cross-pollination between microbiology and drug delivery science.
Questions still open
- Can MTS-derived CPPs deliver therapeutic cargoes like antisense oligonucleotides, CRISPR components, or antibodies with sufficient efficiency for clinical applications?
- What structural features of bacterial MTSs determine endosomal escape ability versus simple cell penetration?
- Could the MTS approach be combined with targeting peptides to achieve cell-type-specific delivery with endosomal escape?
Common questions
What is the endosome trap and why does it matter for drug delivery?
Why use D-amino acid versions of the peptides?
Read the original research
Discovery of endosomalytic cell-penetrating peptides based on bacterial membrane-targeting sequences.
Bioorganic chemistry, 134, 106424
Citation
An, Chuanjing; Wei, Sheng; Dao, Yuankun; Wang, Xiaoya; Dong, Weidong; You, Xue; Tian, Chao; Zhang, Zhili; Dong, Suwei. (2023). Discovery of endosomalytic cell-penetrating peptides based on bacterial membrane-targeting sequences.. Bioorganic chemistry, 134, 106424. https://doi.org/10.1016/j.bioorg.2023.106424