How to Learn Peroxisomal Protein Import: From PTS1 and PTS2 Signals to PEX5/PEX7, Folded Cargo Transport and Receptor Recycling

## Wait, What? Peroxisomes Can Import Proteins That Are Already Folded Most membrane translocases demand an unfolded protein chain. Peroxisomes are different. Many peroxisomal matrix proteins fold in the cytosol first. Some bind cofactors. Some even assemble with partner proteins. They are then imported through a dynamic peroxisomal import system. The two major targeting routes are: > **PTS1 cargo → PEX5 receptor → PEX13/PEX14 import machinery → matrix → PEX5 ubiquitination → PEX1/PEX6 extraction → receptor reuse** and: > **PTS2 cargo → PEX7 + co-receptor → peroxisomal import conduit → matrix → receptor recycling** The 2025 mechanistic advance is especially important: evidence now supports both PTS1 and PTS2 cargo crossing through a selective YG-repeat phase formed by PEX13-related import machinery, while receptor recycling occurs through a separate retrotranslocation system. ## The One-Sentence Answer **Learn peroxisomal matrix-protein import as a receptor-shuttling cycle: PEX5 recognizes C-terminal PTS1 cargo and PEX7 recognizes N-terminal PTS2 cargo with a co-receptor, cargo–receptor complexes cross a selective peroxisomal import conduit that can accommodate folded proteins, cargo is released in the matrix, and ubiquitination plus PEX1/PEX6-driven extraction returns the receptor to the cytosol for another round.** ## Learning Ladder **Beginner:** proteins made in the cytoplasm carry address tags that send them into peroxisomes. **Secondary / Pre-University:** organelles, proteins, amino-acid signals, membranes, ATP and protein folding. **Undergraduate:** PTS1, SKL-like motifs, PTS2, PEX5, PEX7, PEX13, PEX14, PEX2/PEX10/PEX12, PEX1/PEX6 and receptor ubiquitination. **Advanced / Professional:** PEX13 YG-selective phase, WXXXF/Y receptor motifs, PEX7–KPWE interactions, folded-cargo translocation, piggyback import, PEX5 mono-ubiquitination on cysteine, AAA-driven retrotranslocation, deubiquitination and species-specific PTS2 co-receptors. — ## Stage 1: Begin With Peroxisomal Compartmentation Peroxisomes contain enzymes involved in processes such as: – fatty-acid oxidation; – reactive-oxygen metabolism; – ether-lipid synthesis; – specialised metabolism in plants and other eukaryotes. Most matrix proteins are encoded in the nucleus and synthesized on cytosolic ribosomes. The organelle therefore depends on continuous protein import. ## Stage 2: Matrix Import and Membrane-Protein Import Are Different Jobs Peroxisomal membrane proteins often use PEX19/PEX3-related targeting pathways. Matrix proteins use PTS1/PTS2 receptors. This article focuses on **matrix import**. Keeping those routes separate prevents a common trafficking confusion. ## Stage 3: PTS1 Is Usually a C-Terminal Signal The classic peroxisomal targeting signal 1 is a short C-terminal sequence. The best-known example is: > **SKL** Ser-Lys-Leu at the extreme C terminus. Real PTS1 motifs include related variants and surrounding sequence context. ## Stage 4: PEX5 Recognizes PTS1 Cargo PEX5 is a soluble cytosolic receptor. Its C-terminal tetratricopeptide-repeat domain binds PTS1-containing cargo proteins. Its flexible N-terminal region interacts with peroxisomal docking and recycling machinery. PEX5 therefore has separate cargo-recognition and trafficking regions. ## Stage 5: PTS1 Recognition Is Not “SKL or Nothing” Some cargo uses noncanonical PTS1 variants. Affinity depends on: – terminal tripeptide; – upstream residues; – cargo structure. A sequence motif predicts import only probabilistically until receptor binding/localisation is tested. ## Stage 6: PTS2 Is an N-Terminal Signal PTS2 is a different targeting motif located near the N terminus of selected proteins. It is recognized primarily by **PEX7**. The pathway then requires a compatible co-receptor. ## Stage 7: PTS2 Co-Receptors Differ Across Eukaryotes In mammals, PEX7 works with the long isoform of PEX5, often called **PEX5L**. In budding yeast, PEX7 can use specialised co-receptors such as PEX18 and PEX21. The broad logic is conserved while protein architecture differs. ## Stage 8: Cargo Can Fold Before Import Peroxisomes are unusual because matrix cargo need not be fully unfolded. Folded proteins can be imported. This is especially useful for proteins that require cytosolic cofactor loading or assembly before reaching the organelle. ## Stage 9: Some Proteins Can Piggyback Into Peroxisomes A protein lacking its own strong targeting signal can sometimes bind a partner that carries PTS1 or PTS2 information. The complex is imported together. This is called **piggyback import**. Targeting therefore can belong to a protein complex, not only one polypeptide. ## Stage 10: Receptor–Cargo Complexes Dock at the Peroxisomal Membrane PEX5 and PEX7-associated complexes interact with membrane proteins including PEX13 and PEX14. Historically these were described as the docking/import machinery. Modern work increasingly separates docking, selective-phase translocation and receptor retrotranslocation into distinct mechanistic steps. ## Stage 11: PEX13 Contains Tyrosine–Glycine-Rich Sequences PEX13 has a YG-rich region with unusual material properties. This region can form a selective phase reminiscent in principle of FG-repeat nucleoporins in nuclear pores. It creates a striking conceptual link between peroxisomal import and selective polymer barriers. ## Stage 12: PEX5 Contains Motifs That Interact With the YG Phase PEX5 has multiple WXXXF/Y-like motifs in its flexible N-terminal region. These motifs interact with PEX13 YG repeats. The receptor is therefore chemically compatible with a barrier that excludes ordinary proteins. ## Stage 13: The Receptor Can Carry Folded Cargo Through the Selective Phase A useful modern model is: > **PEX5 partitions into PEX13 YG phase → cargo remains bound → receptor drags folded cargo across → complex reaches matrix side** The receptor’s barrier affinity makes folded-cargo translocation possible. ## Stage 14: The Import Conduit Is Not a Permanently Open Protein Pore A stable aqueous hole large enough for folded proteins would threaten membrane integrity. The selective-phase model solves this by creating a dynamic, receptor-compatible transport environment rather than a simple always-open channel. ## Stage 15: 2025 Work Clarified the PTS2 Route A 2025 *Nature Cell Biology* study showed PTS2 cargo–PEX7 complexes also enter peroxisomes through the PEX13 YG phase with suitable co-receptors. This provides a unified physical principle for PTS1 and PTS2 matrix import while preserving distinct cargo-recognition logic. ## Stage 16: PEX7 Uses Additional PEX13 Contacts PTS2 translocation is not simply PEX5 carrying PEX7 passively. PEX7 can interact with a conserved PEX13 motif in the import pathway. This helps explain why PTS2 cargo can traverse the same selective environment with different receptor chemistry. ## Stage 17: Cargo Release and Receptor Recycling Are Different Problems After cargo reaches the matrix, the receptor must return to the cytosol. If PEX5 accumulated permanently inside peroxisomes, import would soon stop. The cell therefore invests ATP in receptor recycling. ## Stage 18: PEX5 Recycling Begins With Mono-Ubiquitination PEX5 is mono-ubiquitinated on a conserved cysteine residue in the normal recycling pathway. This modification acts as a signal for extraction. It is not the same as polyubiquitination targeting a protein for proteasomal destruction. ## Stage 19: The PEX2–PEX10–PEX12 Complex Is the Ubiquitin-Ligase Machinery These peroxisomal membrane proteins form a RING-type ubiquitin-ligase complex. They participate in modifying PEX5 during the export/recycling step. The importer becomes a substrate for an export machine. ## Stage 20: PEX1 and PEX6 Supply ATP-Driven Extraction PEX1 and PEX6 are AAA-family ATPases. They pull ubiquitinated PEX5 out of the peroxisomal membrane/import machinery toward the cytosol. ATP is therefore used mainly for **receptor recycling**, not necessarily for the initial cargo entry step. ## Stage 21: Extraction Can Mechanically Unfold PEX5 Domains Pulling by PEX1/PEX6 can unfold receptor domains during retrotranslocation. That mechanical remodeling helps strip cargo/reorganize the receptor and drive directional recycling. This is a protein-translocation motor acting on the receptor rather than the cargo. ## Stage 22: PEX5 Must Be Deubiquitinated Once returned to the cytosol, PEX5 is deubiquitinated. The receptor can then refold and begin another cargo-binding cycle. A useful full loop is: > **bind cargo → import → ubiquitinate receptor → extract receptor → deubiquitinate → reuse** ## Stage 23: Receptor Recycling Creates Directionality Initial cargo entry can be ATP independent in reconstituted systems. Directionality comes from coupled receptor-state changes and ATP-dependent receptor return. This separates: – cargo passage; – receptor reset. ## Stage 24: PEX7 Recycling Is Mechanistically Distinct PEX7 lacks the same flexible N terminus and conserved cysteine used by PEX5. Its recycling cannot simply be assumed to copy the PEX5 pathway. The 2025 PTS2 work significantly advanced this area, but important details remain active research. ## Stage 25: Failed Receptor Recycling Can Block New Import A receptor trapped at the membrane or in the organelle reduces the available cytosolic receptor pool. Import therefore depends on both entry and reset. A defect in export can appear as a defect in import. ## Stage 26: Peroxisomal Import Can Be Tested With Fluorescent Reporters A fluorescent protein fused to a PTS1 or PTS2 signal can reveal: – diffuse cytosolic localisation; – punctate peroxisomal localisation; – partial import. Localization is powerful but should be combined with biochemical evidence for mechanism. ## Stage 27: Protease Protection Tests Matrix Entry If cargo is inside intact peroxisomes, external protease cannot reach it unless membranes are disrupted. This distinguishes surface association from genuine matrix import. ## Stage 28: Reconstitution Separates Direct Mechanism From Cellular Side Effects Cell-free or purified peroxisomal systems can test: – receptor dependence; – ATP dependence; – PEX13 YG-phase passage; – receptor recycling. This is especially valuable because deleting a PEX gene can alter organelle abundance secondarily. ## Stage 29: Peroxisomal Import Defects Cause Human Disease Defects in peroxins can disrupt peroxisome biogenesis or matrix-protein import. Zellweger-spectrum disorders are major examples of severe peroxisome-biogenesis disease. PTS2-specific defects can produce distinct phenotypes. This article remains mechanistic rather than diagnostic. ## Stage 30: Import Defects and Metabolic Defects Must Be Separated If a peroxisomal enzyme is inactive, possible causes include: – enzyme mutation; – failed targeting; – failed receptor recycling; – missing peroxisome; – cofactor defect. Mechanistic diagnosis begins with the earliest failed layer. ## Stage 31: Peroxisomes Can Import Oligomeric Cargo in Some Contexts The ability to import folded proteins means oligomeric or partner-bound states can sometimes cross. But oligomer size and pathway compatibility vary. “Folded cargo allowed” does not mean unlimited particle size. ## Stage 32: The Nuclear-Pore Analogy Is Useful but Limited Both nuclear pores and the proposed PEX13 conduit use repetitive low-complexity sequences to create selective phases. But their protein compositions, organelle geometries and transport cycles differ. Analogy should illuminate physics without erasing biology. ## Stage 33: The Professional Question Is a Signal–Translocation–Recycle Closure Test Ask: > **Which PTS signal the cargo carries, which receptor recognised it, whether the cargo was folded or complexed before import, how the receptor–cargo complex crossed the PEX13-associated selective phase, whether cargo reached the matrix, how the receptor was ubiquitinated and extracted, and whether the recycled receptor returned to productive cytosolic cargo capture.** ## Evidence: What Proves What? ### Cargo recognition – PTS mutation; – PEX5/PEX7 binding; – structural studies. ### Matrix import – fluorescence localisation; – protease protection; – cell-free import assays. ### Selective-phase mechanism – PEX13 YG-domain studies; – receptor-motif mutants; – hydrogel/reconstitution assays. ### Receptor recycling – PEX5 ubiquitination; – PEX1/PEX6 ATPase perturbation; – deubiquitination assays. ### PTS2 mechanism – PEX7/co-receptor mutants; – 2025 PEX13 interaction studies; – import kinetics. ## Connections Worth Making ### Protein Trafficking Peroxisomal import uses soluble shuttling receptors rather than vesicular transport. ### Protein Folding Unlike Sec-like systems, peroxisomes can import folded matrix proteins. ### Ubiquitin Biology Mono-ubiquitination can function as a reversible trafficking signal rather than a degradation mark. ### AAA ATPases PEX1/PEX6 convert ATP hydrolysis into receptor extraction. ### Selective Polymer Barriers PEX13 YG repeats provide a fascinating conceptual parallel with nuclear-pore FG repeats. ## Misconceptions Worth Hunting – **“All peroxisomal proteins use PTS1.”** PTS2 and other routes exist. – **“PTS1 must be exactly SKL.”** Several variants can function depending on context. – **“PEX5 is a membrane channel.”** It is primarily a soluble shuttling receptor. – **“Peroxisomal cargo must unfold before import.”** Folded cargo can be imported. – **“ATP pushes cargo into the organelle.”** ATP is especially important for receptor recycling through PEX1/PEX6. – **“Ubiquitination always means proteasomal degradation.”** PEX5 mono-ubiquitination is a recycling signal. – **“PTS2 is simply PTS1 with a different peptide.”** It uses PEX7 and distinct co-receptor logic. – **“Matrix import and membrane-protein import are the same pathway.”** They use different targeting machinery. ## Transfer Check A protein ends in a strong PTS1 motif but PEX5 cannot bind it. Is peroxisomal matrix import expected? **No.** PEX5 brings cargo to the membrane but PEX1/PEX6 are inactive. Could later rounds of import decline? **Yes; receptor recycling fails.** A folded enzyme with PTS1 enters intact peroxisomes in a cell-free system. Does that support an unfolded-chain-only translocon model? **No.** PEX7 recognizes a PTS2 cargo but the required co-receptor is absent. Is import complete? **No.** A PEX5 mutant cannot be mono-ubiquitinated on its recycling cysteine. Which stage is most directly impaired? **Receptor extraction/recycling.** ## How We Know the Learning Has Held A learner should be able to distinguish matrix and membrane-protein import; explain PTS1/PEX5; explain PTS2/PEX7/co-receptors; explain folded-cargo compatibility; describe PEX13/PEX14 docking and selective-phase concepts; explain PEX5 ubiquitination; explain PEX2/10/12 and PEX1/PEX6 receptor recycling; distinguish cargo entry from receptor reset; and interpret peroxisomal localisation using mechanistic evidence. ## Model Limits Peroxisomal import differs among animals, fungi and plants. PTS2 usage is reduced or absent in some lineages. The PEX13 YG-selective-phase mechanism is a major recent advance, but native pore architecture and dynamic stoichiometry remain active research. PEX7 recycling is less completely understood than PEX5 recycling. Piggyback import is cargo specific. Peroxisomal membrane-protein insertion is a separate pathway and should not be inferred from matrix-import data. > **Professional peroxisomal-import science keeps cargo folding state + PTS identity + receptor/co-receptor state + PEX13 barrier interaction + matrix delivery + ubiquitination + AAA extraction + receptor reuse visible together.** ## Teaching Guide Teach in this order: **peroxisome function → matrix versus membrane import → PTS1 → PEX5 → PTS2 → PEX7/co-receptor → docking → PEX13 YG phase → folded cargo → cargo release → PEX5 ubiquitination → PEX2/10/12 → PEX1/PEX6 → deubiquitination → PEX7 differences → disease/model limits.** Begin with: > “How can a membrane import a protein that is already folded without opening a permanently leaky pore?” ## Connect This to the eduKate Learning Estate – [Peroxisomes and Glyoxysomes](https://edukatesengkang.com/2026/08/31/how-to-learn-peroxisomes-glyoxysomes/) – [Cell Organelles and Protein Trafficking](https://edukatesengkang.com/2026/08/29/how-to-learn-cell-organelles-protein-trafficking/) – [Protein Folding and Proteostasis](https://edukatesengkang.com/2026/08/29/how-to-learn-protein-folding-proteostasis-amino-acid-sequence-cellular-quality-control/) – [Nuclear Pore Complexes and Nucleocytoplasmic Transport](https://edukatesengkang.com/2026/08/31/how-to-learn-nuclear-pore-complexes-nucleocytoplasmic-transport/) These remain broader or adjacent canonical owners. This article owns **peroxisomal PTS1/PTS2 matrix-protein translocation and receptor recycling**. ## Research Foundations and Further Learning – Reviews of PEX5/PEX7-dependent peroxisomal matrix-protein import. – Structural work on PTS1 recognition by the PEX5 TPR domain. – PEX13/PEX14 docking and selective-barrier studies. – Reconstitution showing PEX13 YG-repeat phases can support PEX5-mediated folded-cargo passage. – PEX2/PEX10/PEX12 ubiquitin-ligase and PEX1/PEX6 receptor-recycling studies. – 2025 *Nature Cell Biology* work defining the PTS2 import mechanism through the PEX13 YG phase and clarifying PEX7/co-receptor interactions. ## The Quiet Ending The beginner asks: “How does a protein know it belongs in a peroxisome?” The developing cell biologist asks: “How can PEX5 carry a folded protein through a membrane barrier?” The advanced learner asks: “Why is ATP spent pulling the receptor back out instead of pushing the cargo in?” And the professional asks: > **Can we close one complete receptor cycle—from cytosolic cargo recognition through selective-phase passage and matrix delivery to ubiquitin-driven receptor extraction and reuse—strongly enough to locate the earliest failure in a peroxisomal import phenotype?**