#peroxisome

Protein Sorting and Trafficking

What is the primary function of Pex11 in peroxisomes?

Peroxisomes are single membrane organelles handling beta oxidation of very long chain fatty acids, alpha oxidation, plasmalogen synthesis and H2O2 detoxification via catalase. Their abundance adapts to metabolic demand through fission of preexisting organelles rather than solely de novo budding from ER. Pex11 family proteins, including Pex11 alpha beta gamma isoforms in mammals, are conserved peroxisomal membrane proteins enriched in regions of high curvature. Upon induction by oleate, fibrates via PPAR alpha, or cold, PEX11 genes transcribed, proteins oligomerize through amphipathic helices and induce membrane elongation forming tubular juxtaposed elongated peroxisomes JEPs. These elongated intermediates acquire fission adaptors Fis1, Mff and tail anchored GDAP1 recruiting dynamin related GTPases Drp1 DLP1 that assemble helical collars constricting membrane in GTP dependent manner for scission. Yeast pex11 deletion yields few enlarged peroxisomes, overexpression produces many small ones. Pex11 does not transport iron, does not act as importomer for matrix proteins which requires Pex5 Pex14, and is unrelated to lysosomal degradation, its dedicated role is membrane remodeling for proliferation.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 13: Peroxisome Biogenesis and Pex11 Division.

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Protein Sorting and Trafficking

Which protein is required for peroxisomal membrane biogenesis?

De novo biogenesis and growth peroxisomal membrane depends targeted insertion peroxisomal membrane proteins PMPs specialized machinery distinct matrix pathway. PMPs including docking Pex14 Pex13 fission factors Pex11 isoforms transporters ABCD1-3 synthesized free cytosolic ribosomes containing hydrophobic transmembrane domains aggregation prone requiring chaperoning. Cytosolic receptor chaperone Pex19 farnesylated C-terminal CAAX box binds PMPs hydrophobic groove preventing aggregation maintaining solubility delivering peroxisomal membrane docking platform. Targeting essential factors Pex3 42 kDa integral membrane short lumenal N-terminus docking anchor Pex19-PMP complex, and Pex16 integral membrane two transmembrane helices recruits Pex3 ER-derived pre-peroxisomal vesicles stabilizes growth. Current model Pex3 Pex16 first insert ER via Sec61 segregate subdomain forming pre-peroxisomal vesicles budding Pex19 dependent mature functional peroxisomes importing matrix enzymes. Pex5 Pex7 soluble matrix import receptors PTS1 PTS2 luminal proteins not membrane biogenesis, Tom20 Tom22 mitochondrial outer receptors presequences, Hsp70 Hsp90 general cytosolic folding chaperones unrelated specific PMP insertion. Human cells lacking Pex3 or Pex16 absence detectable membranes microscopy PMPs mislocalized mitochondria rapidly degraded clinical Zellweger illustrating foundational role membrane formation identity maintenance and disease.

Ref: Distel et al., Annu Rev Biochem: Pex3 and Pex16 essential for peroxisomal membrane biogenesis.

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Protein Sorting and Trafficking

What happens when Pex5 is ubiquitinated by Pex2, Pex10, and Pex12?

Import receptor recycling distinguishes peroxisomes mitochondria requiring extraction shuttling receptor after cargo delivery. After releasing matrix cargo lumen transient pore Pex5 remains embedded peroxisomal membrane peripherally N-terminus facing cytosol partial retrotranslocation. Return cytosol new cycles membrane extraction requires ubiquitination generating handle AAA motor. Specialized peroxisomal E3 ligase complex RING proteins Pex2 Pex10 Pex12 heterotrimeric zinc coordinated embedded membrane associates cytosolic E2 Pex4 anchored membrane Pex22. Pex5 undergoes thioester monoubiquitination conserved cysteine 11 near N-terminus rather than canonical lysine. Monoubiquitinated cysteine provides high-affinity binding site mechanoenzymes Pex1 Pex6 heterohexameric AAA ATPases ring anchored membrane via Pex26 mammals Pex15 yeast consuming ATP thread polypeptide pull Pex5 cytosol. Cytosolic deubiquitinase USP9X removes ubiquitin resetting receptor competent another round. Polyubiquitination lysine residues targets compromised Pex5 proteasomal degradation via RADAR quality control when recycling stalls. Pex5 does not traffic lysosome proteolysis nor mitochondria import nor bind KDEL receptor; fate precisely regulated ubiquitin recycling essential matrix import continuity and organelle functionality maintenance.

Ref: Platta et al., Cell Cycle 2007: Pex5 monoubiquitination by Pex2/10/12 recycles receptor via Pex1/6.

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Protein Sorting and Trafficking

Which receptor recognizes peroxisomal proteins containing a PTS1 signal?

Peroxisomal biogenesis uses two soluble receptors distinguishing two signals. Pex5 70 kDa primary receptor PTS1 pathway handling bulk matrix proteins catalase ACOX1 DBP. Structurally N-terminal half intrinsically disordered contains multiple WXXXF motifs interacting peroxisomal membrane docking protein Pex14 N-terminal domain plus cysteine ubiquitination sites recycling amphipathic helices; C-terminal seven tetratricopeptide repeats TPR 34 aa folding two half-rings superhelical clamp central cavity accommodating C-terminal tripeptide Ser-Lys-Leu water-mediated hydrogen bonds explaining tolerance variant residues. Mechanism stepwise: cytosolic Pex5 binds cargo high affinity escorted peroxisome where N-domain inserts interacting Pex14 Pex13 cargo released matrix through transient pore oligomerized Pex5 Pex14 channel large folded protein passage. Pex5 itself enters lumen transiently then recycled. Pex7 WD40 propeller recognizes PTS2 N-terminal nonapeptide RLX5HL thiolase phytanoyl CoA hydroxylase alkyl DHAP synthase; Pex19 cytosolic chaperone peroxisomal membrane proteins, Rab5 small GTPase marks early endosome fusion unrelated. Defective Pex5 mutations cause Zellweger cytosolic catalase puncta loss very long chain fatty acid accumulation clinically devastating lethal infantile presentation.

Ref: Smith & Subramani, Annu Rev Cell Dev Biol 2019: Pex5 receptor recognizes PTS1 signal.

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Protein Sorting and Trafficking

What type of targeting sequence do peroxisomal matrix proteins contain?

Peroxisomes single-membrane oxidative organelles ubiquitous eukaryotes performing fatty acid alpha beta-oxidation very long chain branched chain detoxifying hydrogen peroxide catalase plasmalogen ether lipid synthesis. Matrix content exceeding 50 enzymes imported post-translationally cytosol free ribosomes. Majority around 90 percent carry peroxisomal targeting signal type 1 PTS1 extreme C-terminus tripeptide (S/A/C)-(K/R/H)-(L/M) canonical most efficient Ser-Lys-Leu derived firefly luciferase discovery. Additional upstream residues modulate affinity receptor. PTS1 recognized tetratricopeptide TPR repeat domain soluble receptor Pex5 forming clamp binding tripeptide nanomolar affinity allowing folded oligomeric proteins cross translocon unique among organelles demonstrated import cross-linked 9 nm particles tetrameric catalase. PTS1 sufficient redirect heterologous non-peroxisomal reporters GFP to peroxisomes. Alternative signals: KDEL retains soluble proteins ER via COPI retrieval, NPXY internalization drives clathrin-mediated endocytosis via PTB adaptors Dab2 ARH linking AP2, DXE directs ER exit COPII vesicles Sec24 binding. Mutation deletion SKL abolishes import causing cytosolic mislocalization as Zellweger spectrum disorders with very long chain fatty acid accumulation clinical severity and developmental defects.

Ref: Lodish et al., MCB: PTS1 SKL tripeptide targets matrix proteins to peroxisomes.

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