#signal sequence

Cell Organelles

The N-terminal signal sequence of secretory proteins is recognized by:

Targeting of secretory proteins to endoplasmic reticulum uses signal hypothesis where N-terminal fifteen to thirty amino acid hydrophobic sequence emerges from ribosome exit tunnel exposing binding site for signal recognition particle, a three hundred kDa ribonucleoprotein comprising seven SL RNA and six protein subunits including SRP54 that contains methionine-rich pocket accommodating hydrophobic signals via induced fit. Binding transiently arrests elongation, complex diffuses to ER membrane where heterodimeric SRP receptor composed of SR alpha and SR beta, both GTPases, docks particle. GTP hydrolysis by SRP54 and SR alpha drives transfer of signal sequence to Sec61 translocon channel, opening laterally for membrane integration. Ribosome then resumes translation translocating nascent chain into lumen. SNARE proteins mediate post-targeting vesicle fusion using coiled-coil zippering, clathrin forms lattice coats during endocytosis, ribophorin anchors ribosome but does not recognize signal. Genetic ablation in bacteria Ffh or yeast SRP54 leads to mislocalization of secreted proteins aggregating in cytosol, confirming essential sorting role preventing toxic accumulation.

Ref: Walter Nature 1981 SRP discovery; SRP54 binds signal peptide, GTP-dependent delivery to Sec61.

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Protein Sorting and Trafficking

Which signal is necessary for targeting soluble lysosomal enzymes?

Among diverse sorting determinants mannose-6-phosphate uniquely marks entire class of soluble lysosomal precursors enabling intracellular lysosome biogenesis. Newly synthesized acid hydrolases enter ER lumen, receive high-mannose N-glycans via oligosaccharyltransferase, fold with calnexin, move to cis-Golgi where Golgi-resident phosphotransferase adds M6P as phosphodiester then uncovered in TGN to yield exposed phosphate. Carbohydrate address recognized in TGN pH 6.7 by two M6P receptors both P-type lectins: 300 kDa cation-independent IGF2 receptor binding also IGF2 and 46 kDa cation-dependent requiring divalent cation. Receptors cluster via cytosolic motifs YXXPhi and DXXLL binding AP1 and GGA recruiting clathrin lattice. KDEL retrieval maintains ER chaperones BiP via KDEL receptor and COPI, DXE mediates ER exit for membrane cargo via Sec24 COPII, NPXY plus AP2 handles plasma membrane endocytosis via ARH co-adaptor. Without M6P addition or receptor, lysosomal enzymes follow default secretion to extracellular medium. Experimental addition exogenous M6P competes for receptor binding abolishing sorting confirming receptor-ligand nature essential for lysosome formation and cell physiology.

Ref: Lodish et al., MCB: Mannose-6-phosphate signal targets soluble lysosomal enzymes to lysosomes.

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Protein Sorting and Trafficking

Which domain of SRP binds to signal sequences on nascent proteins?

Specificity of signal recognition particle for hydrophobic targeting signals resides within C-terminal M domain of 54 kilodalton subunit SRP54. Structural analyses of bacterial homolog Ffh and mammalian SRP54 show M domain folds into deep groove lined almost exclusively with methionine side chains whose flexible thioether and long aliphatic chain create plastic hydrophobic bristle adaptable to varied signal sequence compositions and lengths. Basic residues surrounding groove interact with phosphate backbone of 7SL RNA and ribosomal proteins L23 and L29 near peptide exit tunnel. Adjacent NG GTPase domains dimerize with SRP receptor SRα via GTP-dependent interaction but do not contact signal directly. Upon signal accommodating as alpha-helix inside groove, conformational change extends to linker connecting M and NG domains, signaling Alu domain to pause translation. ATPase or BiP domains absent from SRP. This methionine-rich architecture explains how single particle binds hundreds of diverse ER targeting signals with high affinity yet promiscuous selectivity, ensuring efficient capture of secretory proteins early during synthesis preventing cytosolic mislocalization, aggregation and degradation by proteasome quality control and maintaining secretory flux.

Ref: Keenan et al., Annu Rev Biochem 70: 2001, SRP54 M Domain Binds Signal Sequences.

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Protein Sorting and Trafficking

The signal sequence of a nascent protein is cleaved by:

N-terminal signal peptides exhibit tripartite organization essential for targeting and cleavage: positively charged n-region with one to three basic residues, hydrophobic core h-region of seven to twelve aliphatic residues forming alpha-helix, and polar c-region containing signal peptidase recognition motif Ala-X-Ala at positions minus three and minus one relative to cleavage site, often followed by small residues. Once nascent chain enters ER lumen through Sec61 channel, signal peptidase complex anchored on lumenal face performs co-translational cleavage. Heteromeric complex includes two catalytic subunits SEC11A and SEC11C that are serine proteases of S26 family using Ser-His-Asp catalytic triad, plus accessory subunits SPCS1, SPCS2, SPCS3 stabilizing assembly and positioning active site near membrane interface. Catalytic serine attacks carbonyl after c-region, hydrolyzing bond and liberating mature protein from membrane-tethered signal. Cleaved signal peptides further degraded by intramembrane signal peptide peptidase. Sec61 itself lacks proteolytic activity, SRP receptor only delivers, translocon pore conducts, so efficient cleavage is prerequisite for subsequent folding, N-glycosylation and trafficking beyond ER, preventing retention as membrane-anchored precursor that could disrupt membrane integrity.

Ref: Paetzel et al., Chem Rev 102: 2002, Signal Peptidase Cleaving Signal Sequences.

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Protein Sorting and Trafficking

Which of the following recognizes the signal sequence of nascent proteins targeting the ER?

Early sorting of nascent secretory and membrane proteins is performed by signal recognition particle, conserved ribonucleoprotein containing 7SL RNA scaffold and six protein subunits. When hydrophobic signal sequence of about eight to twelve non-polar residues emerges from ribosomal exit tunnel, methionine-rich M domain of SRP54 forms flexible hydrophobic groove whose abundance of sulfur-containing methionine side chains allows plastic accommodation of diverse signal sequences via induced fit. Binding is communicated through 7SL RNA to Alu domain comprised of SRP9 and SRP14 heterodimer that docks at elongation factor binding site, temporarily pausing translation to prevent premature folding and aggregation in cytosol. SRP-ribosome-nascent chain complex then diffuses to ER where GTP-dependent interaction with heterodimeric SRP receptor made of SRα and SRβ GTPases delivers complex to Sec61 channel for hand-off. Ran-GTP controls nuclear import via importins, KDEL receptor retrieves escaped ER chaperones via COPI, Rab GTPases govern vesicle tethering and fusion specificity, none directly scan ribosome exit tunnel for hydrophobic nascent signals at this early checkpoint of protein sorting to ER lumen.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 13: SRP Recognizes Signal Sequences.

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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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