#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 me

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 monoubiquiti

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 toleran

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

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 p

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

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, larg

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 d

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

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