Protein Sorting and Organelle Targeting Mechanisms

Vocabulary practice flashcards covering protein targeting, translocation machinery, sorting pathways, and experimental analyses across peroxisomes, mitochondria, the endoplasmic reticulum, and the nucleus.

PTS1 (Peroxisomal Targeting Signal 1)

A peroxisomal targeting sequence located at the C-terminus of matrix proteins, typically ending in Ser-Lys-Leu (SKL), that is recognized by the cytosolic receptor PEX5.

PTS2 (Peroxisomal Targeting Signal 2)

An alternative targeting sequence for peroxisomal matrix proteins that is recognized by the soluble targeting receptor PEX7 in conjunction with PEX5.

PEX5

A soluble cytosolic cycling receptor that recognizes C-terminal PTS1 signals on folded cargo and engages the PEX13-containing import machinery at the peroxisomal membrane.

PEX7

A soluble cytosolic receptor that specifically recognizes PTS2-containing cargo proteins and mediates their delivery into the peroxisomal matrix alongside PEX5.

PEX19

A cytosolic factor that recognizes membrane-targeting signals on peroxisomal membrane proteins and delivers them to PEX3 for bilayer insertion.

PEX3

An integral peroxisomal membrane protein that receives membrane-destined proteins from PEX19 to facilitate their insertion into the peroxisomal membrane.

PEX2–PEX10–PEX12 Complex

A peroxisomal membrane complex that modifies the PEX5 receptor after cargo release into the organelle, preparing PEX5 for export and recycling.

PEX1–PEX6 ATPase

An ATP-driven motor complex that extracts modified PEX5 from the peroxisomal membrane back into the cytosol so it can be reused.

PEX13

A peroxisomal membrane component of the import machinery that the cargo–receptor complex engages during matrix protein import.

Firefly Luciferase PTS1 Deletion Experiment

An experiment showing that removing the C-terminal SKL signal from peroxisomal luciferase shifts its localization from punctate peroxisomal fluorescence to diffuse cytosolic fluorescence, demonstrating that PTS1 is necessary for normal targeting.

DHFR–PTS1 Fusion Experiment

An experiment showing that appending a C-terminal PTS1 sequence to cytosolic DHFR converts its diffuse cytosolic localization to punctate peroxisomal fluorescence, proving that PTS1 is sufficient for peroxisomal targeting.

Mitochondrial Presequence

An N-terminal targeting signal forming an amphipathic alpha helix with positively charged and hydrophobic residues that directs matrix-destined proteins into mitochondria based on a chemical pattern rather than a fixed sequence.

TOM Complex

The translocase of the outer mitochondrial membrane that recognizes presequences and translocates unfolded polypeptides across the outer membrane.

Tom40

The core pore-forming channel subunit of the mitochondrial TOM complex in the outer membrane through which presequence-containing proteins pass.

TIM23 Complex

An inner mitochondrial membrane translocase that moves matrix-destined proteins across the inner membrane and releases proteins with stop-transfer segments laterally into the inner membrane.

Mitochondrial Inner-Membrane Potential

An electrical gradient across the mitochondrial inner membrane (negative inside, positive outside) that electrophoretically pulls positively charged presequences into the matrix space.

Mitochondrial Hsp70 (mtHsp70)

A matrix molecular chaperone that binds emerging polypeptide chains and uses ATP-driven conformational cycles to pull them through TIM23 into the matrix.

TIM22 Complex

An inner mitochondrial membrane translocase that inserts multipass carrier proteins containing internal targeting signals into the inner membrane using the membrane potential.

Intermembrane-Space Chaperones

Soluble proteins in the mitochondrial intermembrane space that bind hydrophobic precursor segments exiting the TOM complex to prevent aggregation and escort them to TIM22.

OXA Route

A pathway mediated by the OXA complex in the inner mitochondrial membrane that inserts mitochondrially encoded proteins from inside the matrix into the inner membrane.

Import Then Re-insert Route

A mitochondrial pathway in which nuclear-encoded proteins first enter the matrix, have their initial targeting signal cleaved, and then use a second signal sequence to insert into the inner membrane via the OXA complex.

Methotrexate (DHFR Import Assay)

A drug that tightly binds and stabilizes the folded structure of DHFR; in mitochondrial import experiments, it permits presequence cleavage but blocks translocation through import channels, demonstrating that matrix import requires proteins to unfold.

ER Signal Sequence

A hydrophobic amino acid sequence that directs translating ribosomes and nascent chains to the ER membrane for co-translational translocation or membrane insertion.

Signal Recognition Particle (SRP)

A cytosolic ribonucleoprotein complex that recognizes the hydrophobic ER signal on a nascent chain, pauses translation, and targets the ribosome to the ER membrane.

SRP Receptor

A membrane receptor on the ER that binds the SRP–ribosome complex in a GTP-regulated manner to transfer the nascent chain to the Sec61 translocator.

Sec61 Translocator

The protein-conducting channel in the ER membrane that mediates the co-translational entry of soluble proteins into the ER lumen or the lateral insertion of transmembrane segments into the bilayer.

Stop-Transfer-Anchor (STA) Sequence

A hydrophobic segment within a translocating polypeptide that halts transfer through the Sec61 translocator and is released laterally into the lipid bilayer as a transmembrane domain.

Signal Peptidase

An ER lumenal enzyme that cleaves the N-terminal signal sequence from a translocating polypeptide chain during entry into the ER.

Nuclear Pore Complex (NPC)

A large multi-protein structure spanning the nuclear envelope that mediates the selective, bidirectional transport of folded macromolecules between the cytosol and the nucleus.

FG Repeats

Phenylalanine–glycine-rich sequence repeats in channel nucleoporins that form a flexible, selective meshwork within the nuclear pore complex, interacting weakly and transiently with nuclear transport receptors.

Importins

Soluble nuclear transport receptors that bind nuclear localization signals on folded cargo and interact with FG repeats to facilitate translocation through the nuclear pore complex.

Nuclear Localization Signal (NLS)

A peptide targeting sequence on folded cargo proteins that directs their import through nuclear pore complexes into the nucleus.

Ran-GEF

A nuclear-localized enzyme that promotes the exchange of GDP for GTP on Ran, maintaining a high concentration of Ran-GTP inside the nucleus.

Ran-GAP

A cytosolic enzyme that stimulates Ran to hydrolyze GTP to GDP, maintaining a high concentration of Ran-GDP in the cytosol.

Ran-GTP/Ran-GDP Asymmetry

The compartmentalized concentration gradient of Ran (high Ran-GTP in the nucleus, high Ran-GDP in the cytosol) that drives the directionality of nucleocytoplasmic transport and cargo release.

Nucleoplasmin Experiment

An experiment demonstrating that nucleoplasmin tail fragments accumulate in the nucleus while head fragments remain in the cytoplasm, showing that targeting information resides within the tail region.

SV40 Large T-Antigen

A viral protein used as a classical model system to demonstrate that nuclear localization signals are necessary and sufficient for nuclear translocation.

XPO1 (Exportin-1)

A nuclear export receptor that transports proteins including tumor suppressors p53 and p27 from the nucleus to the cytoplasm; often upregulated in tumor cells, causing abnormal cytoplasmic mislocalization.

Selinexor

A small-molecule inhibitor of XPO1 that blocks the nuclear export of tumor suppressor proteins, restoring their nuclear retention and normal functional activity in tumor cells.