#protein import

Protein Sorting and Trafficking

What is the main function of the TIC complex in chloroplasts?

Chloroplast envelope encloses two translocon systems tandem importing majority organelle proteome 3000 proteins despite plastid genome about 100. Precursors synthesized cytosol N-terminal transit peptide enriched hydroxylated serine threonine positively charged amphipathic not hydrophobic distinct mitochondrial presequence. First receptor TOC outer envelope: Toc159 large 159 kDa GTPase selectivity filter photosynthetic versus housekeeping via acidic A-domain, Toc34 small 34 kDa regulatory, Toc75 Omp85 beta-barrel superfamily 14-strand channel 3 nm pore translocating polypeptide. After outer membrane TOC hands substrate TIC inner envelope: Tic110 main cation-selective channel large intermembrane space domain recruiting peptide stromal scaffold binding chaperones, Tic40 co-chaperone Sti1-like TPR recruiting Hsp93 ClpC AAA ATPase ATP-driven motor pulling substrate, Tic20 alternative smaller channel, Tic22 soluble intermembrane bridging, Tic236 giant forming supercomplex spanning both membranes creating direct continuous contact sites. Upon arrival stroma transit peptide cleaved stromal processing peptidase SPP metalloenzyme producing mature protein. TIC does not import across outer envelope TOC function does not direct Golgi plastids lack connections nor recycle; exclusive role inner envelope passage stromal compartment preceding further thylakoid sorting Sec SRP Tat pathways essential development and photosynthesis.

Ref: Soll & Schleiff, Nat Rev Mol Cell Biol 2004: TIC imports proteins across chloroplast inner envelope.

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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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Protein Sorting and Trafficking

What is the function of the OXA complex in mitochondria?

Evolutionary conservation membrane insertases bacterial plasma membrane reveals OXA family YidC Alb3 Oxa1. Mitochondrial Oxa1 45 kDa inner membrane five transmembrane helices forms co-translational insertion site adjacent matrix mitoribosome large subunit near exit tunnel via Mrpl45 interaction. Several highly hydrophobic proteins encoded mitochondrial DNA including subunit II III cytochrome oxidase Cox2 Cox3 subunit a c ATP synthase Atp6 Atp9 cytochrome b Cob synthesized matrix mitoribosomes because extreme hydrophobicity would hamper import. Oxa1 central hydrophilic groove provides aqueous path lateral gate opening lipid bilayer facilitating insertion nascent helices using positive inside rule proton motive force independent ATP. Nuclear-encoded proteins first fully imported matrix via TIM23 such Cox18 dependent and some ATP synthase subunits also require subsequent export inner membrane conservative sorting analogous Sec-independent insertion again via Oxa1. Oxa1 does not insert beta-barrel outer proteins relying SAM complex Tob55 nor directly drive matrix import via TIM23 nor act proton pump; Complexes I III IV pump protons. Deletion Oxa1 yeast pleiotropic respiratory deficiency failure assembling Complexes IV V loss membrane potential and biogenesis.

Ref: Hennon et al., Front Physiol 2015: OXA insertase inserts proteins into mitochondrial inner membrane.

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Protein Sorting and Trafficking

The nuclear localization signal (NLS) is typically enriched in:

Classical nuclear localization signals are defined chemically by clustering of positively charged side chains within short peptide stretch. Monopartite type contains four to five consecutive basic residues such as Lys-Lys-Lys-Arg-Lys originally identified in SV40 large T antigen, while bipartite type features two basic clusters separated by ten to twelve residues, exemplified by nucleoplasmin KRPAATKKAGQAKKKK and many transcription factors. Arginine guanidinium and lysine ammonium groups provide both electrostatic attraction and hydrogen bonding to acidic and aromatic residues lining major and minor grooves of importin-alpha armadillo repeats. Systematic alanine scanning shows replacement of basic residues abolishes nuclear accumulation, whereas fusion of polybasic sequence confers constitutive nuclear targeting to cytosolic reporters like pyruvate kinase. Leucine-rich sequences define export signals interacting with CRM1, tyrosine-based YXXΦ motifs mediate clathrin endocytosis, methionine-rich regions mark SRP binding, so enrichment in arginine and lysine uniquely marks NLS distinguishing import information, with phosphorylation adjacent modulating affinity during cell cycle, stress and signal transduction to control nuclear availability of regulators and transcription factors. Additional coordination with cellular stress pathways ensures fidelity, prevents aggregation, and links trafficking to growth control and proteostasis maintenance across diverse cell types and developmental stages.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 13: Basic Residues in Classical NLS.

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Protein Sorting and Trafficking

Which transport mechanism does NOT require a signal sequence?

Small metabolites, ions, nucleotides and proteins smaller than roughly 40 kilodaltons equilibrate between nucleus and cytoplasm without peptide targeting signals because the nuclear pore complex forms a large aqueous channel about nine nanometers wide lined with intrinsically disordered FG-repeat nucleoporins that act as a size-selective hydrogel sieve allowing diffusion along concentration gradients. This passive process supports rapid exchange of ATP, GTP, amino acids and small second messengers needed for transcription, DNA replication and growth, without consuming energy. By contrast, larger macromolecules exceeding the diffusion limit rely on active receptor-mediated transport. Mitochondrial proteins use amphipathic N-terminal presequences recognized by TOM20 receptor, ER secretory proteins carry hydrophobic signal peptides bound by signal recognition particle, and ER-Golgi trafficking requires coat-dependent capture via di-acidic or KDEL retrieval motifs. NLS enriched in lysine and arginine and leucine-rich NES recognized by importin and exportin families provide specificity for nuclear crossing. Therefore nuclear flux of small species represents receptor-independent equilibration that conserves energy while permitting continuous communication between genome and cytosolic metabolism essential for cell cycle progression, signal transduction and maintenance of nucleotide pools across compartments.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 12: Nuclear Pore Complex and Passive Diffusion of Small Molecules.

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Protein Sorting and Trafficking

Which targeting signal is required for protein import into the mitochondria?

Majority of mitochondrial proteins are encoded by nuclear genome synthesized in cytosol as precursors requiring active import. Matrix targeting signal resides typically at extreme N-terminus forming 15 to 70 amino acid extension lacking acidic residues and capable of adopting amphipathic alpha-helical conformation with positively charged lysine and arginine residues clustered on one face creating basic patch and hydrophobic leucine, phenylalanine on opposite face. This structure is recognized by receptor domains of translocase of outer membrane Tom20 with hydrophobic groove plus Tom22 acidic domain interacting with basic face. Precursor traverses Tom40 beta-barrel channel then outer to inner space, guided by Tim50 and translocase of inner membrane TIM23 complex where membrane potential Delta psi across inner membrane exerts electrophoretic attraction on positively charged residues and matrix Hsp70 ATP hydrolysis pulls polypeptide inward. Upon import, mitochondrial processing peptidase MPP cleaves presequence and chaperonin Hsp60 facilitates folding. Mutations disrupting amphipathicity cause mitochondrial import failure and disease phenotypes like Mohr-Tranebjaerg syndrome.

Ref: Neupert & Herrmann, Annu Rev Biochem 2007, Mitochondrial Protein Import. Alberts 7th ed., Chapter 12.

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