Beyond Membrane Disruption: AMPs That Hit Inside

AMP Mechanisms

2 Classes of Ribosome Blockers

Proline-rich antimicrobial peptides kill bacteria through two distinct ribosome-binding mechanisms: blocking protein elongation and disrupting translation termination.

Seefeldt et al., Nucleic Acids Research, 2016

Seefeldt et al., Nucleic Acids Research, 2016

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The textbook version of how antimicrobial peptides kill bacteria is straightforward: they punch holes in the membrane and the cell contents leak out. That model is accurate for many AMPs, but it is incomplete. A growing body of research demonstrates that some antimicrobial peptides cross bacterial membranes without destroying them and kill by attacking intracellular targets including DNA, RNA, ribosomes, and protein-folding chaperones. For the membrane disruption mechanisms, see our pillar article on how antimicrobial peptides kill bacteria.

These intracellular-targeting AMPs are significant for drug development because they offer multi-hit killing mechanisms that are harder for bacteria to resist. A bacterium can potentially modify its membrane to evade a pore-forming peptide, but simultaneously mutating its ribosomes, DNA-binding proteins, and chaperone systems to evade an intracellular-targeting AMP is a much taller evolutionary order.

How AMPs Get Inside Without Breaking the Door

The defining feature of intracellular-targeting AMPs is membrane translocation without lysis. These peptides use the bacterial membrane as a passageway, not a target. Several mechanisms have been identified:

Self-promoted uptake: Some cationic AMPs interact with the lipopolysaccharide (LPS) layer of gram-negative bacteria, displacing divalent cations (Mg2+ and Ca2+) that stabilize the outer membrane. This transiently increases permeability enough for the peptide to cross without forming stable pores. The membrane reseals behind the peptide.

Transporter hijacking: Proline-rich AMPs exploit bacterial inner membrane transporters, particularly the SbmA transporter in gram-negative bacteria, to gain entry. This is an energy-dependent, receptor-mediated process entirely different from brute-force membrane disruption. Bacteria that lack functional SbmA transporters are resistant to PrAMPs, confirming the transporter's role in uptake.

Buforin II's proline hinge: Buforin II, a 21-amino-acid peptide derived from the stomach protein histone H2A of the Asian toad Bufo bufo gargarizans, contains a proline residue at position 11 that creates a flexible hinge in its alpha-helical structure. This hinge allows the peptide to traverse the membrane in a "flip" mechanism without forming a stable pore. Electrophysiology studies confirmed that buforin II crosses lipid bilayers without producing the conductance changes characteristic of pore formation.

Ribosome Targeting: The Proline-Rich AMPs

The best-characterized intracellular AMP mechanism is ribosome inhibition by proline-rich antimicrobial peptides (PrAMPs). These peptides are found across invertebrate species, including insects, crustaceans, and recently identified in ruminant mammals.^[1]

Structural studies using cryo-electron microscopy and X-ray crystallography have revealed that PrAMPs bind within the polypeptide exit tunnel (PET) of the bacterial 70S ribosome, near the peptidyl transferase center where amino acids are joined into proteins. Two distinct classes of PrAMPs have been identified based on their binding mechanisms:

Class I PrAMPs (Bac7, Onc112, pyrrhocoricin, metalnikowin) bind the ribosomal exit tunnel and block the delivery of aminoacyl-tRNA by elongation factor Tu (EF-Tu) to the ribosomal A-site. This stalls translation during the elongation phase. The ribosome sits on the mRNA with a partially built protein, unable to add the next amino acid.

Class II PrAMPs (apidaecin 1b, Api137) act during translation termination. They inhibit protein synthesis by trapping release factors on the 70S ribosome, preventing the ribosome from releasing the completed protein and recycling for the next round of translation. A 2024 cryo-EM study revealed that Api137 occupies a second binding site near the exit of the PET and can repress translation independently of release factor trapping, indicating the mechanism is more complex than initially understood.

The recently discovered rumicidins from ruminant mammals represent a new addition to ribosome-targeting AMPs. Panteleev et al. (2024) showed these peptides kill bacteria by plugging the ribosome exit tunnel, with broad-spectrum activity against both gram-positive and gram-negative pathogens and no toxicity to human cells.^[1] The discovery of mammalian PrAMPs was unexpected, as this mechanism was previously thought to be confined to invertebrates.

DNA and RNA Targeting

Several AMPs kill bacteria by directly interfering with nucleic acid function:

Buforin II is the paradigmatic example. After crossing the membrane via its proline-hinge mechanism, buforin II binds both DNA and RNA with high affinity. The binding inhibits replication, transcription, and translation simultaneously. Computational studies have engineered buforin II variants with increased DNA affinity by modifying electrostatic surface properties, demonstrating that DNA binding is both the primary killing mechanism and a tunable design parameter.

Microcin J25 is a 21-amino-acid lasso peptide produced by E. coli that inhibits bacterial RNA polymerase. Its unusual lasso topology, in which the C-terminal tail threads through a macrolactam ring formed by the N-terminus, makes it extremely resistant to proteolytic degradation. Microcin J25 enters target bacteria through the iron transporter FhuA and binds to the beta-prime subunit of RNA polymerase, blocking substrate access to the catalytic center.^[2] This is mechanistically similar to the antibiotic rifampicin, but microcin J25 binds at a different site, meaning bacteria resistant to rifampicin remain susceptible to the peptide.

Indolicidin, a 13-amino-acid tryptophan-rich peptide from bovine neutrophils, inhibits DNA synthesis at concentrations below its membrane-lytic threshold. At sub-lytic concentrations, indolicidin crosses the membrane and binds DNA in the minor groove, interfering with topoisomerase activity and DNA replication. At higher concentrations, membrane disruption dominates. This dose-dependent switch between intracellular and membrane mechanisms is common among AMPs and complicates the classification of "intracellular" versus "membrane" peptides.

Chaperone Targeting: Disrupting Protein Quality Control

A third intracellular mechanism involves targeting molecular chaperones, the protein-folding machines that maintain protein homeostasis in bacterial cells.

DnaK targeting: Pyrrhocoricin, drosocin, and apidaecin bind to DnaK, the major bacterial Hsp70 chaperone. DnaK assists in folding newly synthesized proteins and refolding damaged ones. When these AMPs bind DnaK's substrate-binding domain, they compete with the chaperone's natural protein clients. The result is accumulation of misfolded proteins, proteotoxic stress, and cell death.

GroEL targeting: Some AMPs also interact with GroEL, the bacterial chaperonin that forms a barrel-shaped chamber for protein folding. Disruption of GroEL function is particularly damaging because it handles the folding of approximately 10 to 15% of all cytoplasmic proteins, including many essential enzymes.

The chaperone-targeting mechanism is attractive for drug development because DnaK and GroEL have no close human homologs in terms of substrate specificity. Human Hsp70 and Hsp60 perform analogous functions but differ enough in their substrate-binding domains that selectivity is achievable.

Why Multi-Target Killing Matters for Resistance

The most compelling argument for intracellular-targeting AMPs is that multi-target killing creates a higher barrier to resistance evolution. For antimicrobial peptides as antibiotic alternatives, this property is central to their therapeutic promise.

Consider the resistance challenge from the bacterial perspective:

• To resist a pore-forming AMP, a bacterium can modify its membrane lipid composition (a single adaptation)
• To resist an AMP that targets both the membrane and ribosomes, it must simultaneously modify its membrane and its ribosomal exit tunnel (two independent adaptations)
• To resist a multi-hit AMP like those that affect membranes, DNA binding, and chaperone function, three or more independent mutations are needed concurrently

The probability of developing resistance scales exponentially with the number of independent targets. This is the same principle behind combination antibiotic therapy, but encoded in a single molecule.

Laboratory evolution experiments support this model. Bacteria develop resistance to single-mechanism antibiotics readily (often within days to weeks of serial passage), while resistance to multi-mechanism AMPs develops much more slowly, if at all, in comparable experimental timeframes.

Dual-Mechanism Peptides: The Gray Zone

Many AMPs do not fit neatly into "membrane" or "intracellular" categories. Cathelicidins like LL-37 disrupt membranes at high concentrations but also cross membranes at lower concentrations and interact with intracellular targets including DNA.^[3] The dominant killing mechanism depends on peptide concentration, bacterial species, growth phase, and environmental conditions.

This duality complicates research but reflects the evolved function of natural AMPs. In the immune system, peptide concentrations vary by orders of magnitude depending on proximity to the infection site. A peptide that kills by membrane disruption at high local concentrations near a neutrophil degranulation event and by intracellular targeting at lower concentrations further from the source provides layered antimicrobial protection.

The practical implication for drug design is that understanding the full mechanism spectrum of an AMP candidate is essential. A peptide selected for membrane-lytic activity in high-concentration assays might actually kill through intracellular mechanisms at the concentrations achievable in vivo. Missing this distinction leads to incorrect structure-activity relationships and suboptimal engineering decisions.

For context on why AMPs target bacterial membranes but spare your own cells, the selectivity mechanisms differ between membrane-active and intracellular-active AMPs. Membrane selectivity depends on charge differences between bacterial and mammalian cell surfaces. Intracellular selectivity depends on transporter specificity (SbmA is bacterial-specific) and target divergence (bacterial ribosomes differ structurally from eukaryotic ribosomes). Understanding where selectivity comes from determines which engineering approaches can improve therapeutic indices. Researchers working on bee venom peptides like melittin face similar challenges in separating therapeutic activity from toxicity.

The Bottom Line

Antimicrobial peptides that target intracellular processes represent a mechanistically distinct class from membrane-lytic AMPs. Proline-rich AMPs block bacterial protein synthesis by binding the ribosomal exit tunnel through two distinct mechanisms. Buforin II and related peptides kill by binding DNA and RNA after non-lytic membrane translocation. Others target protein-folding chaperones DnaK and GroEL. The multi-target nature of many intracellular AMPs creates higher barriers to resistance evolution than single-mechanism antibiotics. Most AMPs operate across a spectrum of mechanisms depending on concentration, making rigid classification less useful than understanding the full activity profile of each peptide.

Frequently Asked Questions

How do antimicrobial peptides kill bacteria without destroying the membrane?

Some AMPs cross bacterial membranes without forming permanent pores and kill by targeting intracellular processes. Proline-rich AMPs enter through bacterial membrane transporters (like SbmA) and block ribosomal protein synthesis. Buforin II uses a proline-hinge mechanism to translocate across the membrane and binds DNA and RNA inside the cell. Microcin J25 enters through iron transporters and inhibits RNA polymerase.

What intracellular targets do antimicrobial peptides attack?

Known intracellular targets include bacterial ribosomes (blocked by proline-rich AMPs like Bac7 and apidaecin), DNA and RNA (bound by buforin II and indolicidin), RNA polymerase (inhibited by microcin J25), and protein-folding chaperones DnaK and GroEL (disrupted by pyrrhocoricin, drosocin, and apidaecin). Some AMPs target multiple intracellular systems simultaneously.

Can bacteria become resistant to intracellular-targeting AMPs?

Resistance develops more slowly against multi-target AMPs than against single-target antibiotics. A bacterium must simultaneously mutate multiple independent targets (ribosomes, DNA-binding proteins, chaperones) to achieve resistance, and the probability of concurrent mutations is exponentially lower than for single-target adaptation. Laboratory evolution experiments confirm that resistance to multi-mechanism AMPs develops much more slowly than to conventional antibiotics.

What are proline-rich antimicrobial peptides?

Proline-rich antimicrobial peptides (PrAMPs) are a class of AMPs found in insects, crustaceans, and ruminant mammals that kill bacteria primarily by inhibiting protein synthesis at the ribosome. They enter bacteria through inner membrane transporters rather than by disrupting membranes. Two classes exist: Class I blocks protein elongation by preventing aminoacyl-tRNA delivery, while Class II traps release factors during translation termination.

What is buforin II and how does it work?

Buforin II is a 21-amino-acid antimicrobial peptide derived from the stomach protein histone H2A of the Asian toad. Unlike most AMPs, it crosses bacterial membranes without forming pores, using a proline residue at position 11 that creates a flexible hinge in its structure. Once inside, buforin II binds DNA and RNA with high affinity, inhibiting replication, transcription, and translation simultaneously.

Are intracellular AMPs being developed as antibiotics?

Intracellular-targeting AMPs are in preclinical development as antibiotic candidates. Their multi-target killing mechanisms and low resistance potential make them attractive. Challenges include production cost, in vivo stability, and achieving sufficient intracellular concentrations at infection sites. No intracellular-targeting AMP has completed Phase III clinical trials as of 2026, though the discovery of mammalian PrAMPs (rumicidins) in 2024 expanded the candidate pool.