Cell Biology – Protein Sorting II Vocabulary

30 vocabulary flashcards reviewing key terms and mechanisms related to endosymbiosis, mitochondrial, peroxisomal, and nuclear protein targeting from Cell Biology Lecture 9–11.

Endosymbiosis Theory

The proposal that mitochondria and chloroplasts originated when an ancestral eukaryotic cell engulfed bacteria that became permanent, mutually beneficial residents.

Mitochondrial DNA

The small circular genome inside mitochondria whose bacterial-like sequences support their endosymbiotic origin.

Chloroplast Genome

DNA found within chloroplasts; closely resembles cyanobacterial DNA and underpins the endosymbiosis model.

Protein Sorting

The cellular process that directs newly made proteins to their correct destinations using organelle-specific signal sequences.

Signal Sequence

A short amino-acid motif that functions as a molecular address label, guiding proteins to a particular organelle or membrane.

Amphipathic Helix

An α-helix with one hydrophobic face and one positively charged face; characteristic of mitochondrial matrix-targeting signals.

Matrix-Targeting Sequence (MTS)

N-terminal amphipathic helix (≈20–50 residues) that directs proteins into the mitochondrial matrix and is cleaved after import.

Tom Complex

‘Translocon of the Outer Membrane’; receptor/channel system that recognizes and transports precursor proteins across the mitochondrial outer membrane.

Tim Complex

‘Translocon of the Inner Membrane’; inner-membrane channels (Tim23/17, Tim22, Tim44, etc.) that convey proteins into the matrix or embed them in the inner membrane.

General Import Pore (Tom40)

The β-barrel channel within the Tom complex through which unfolded mitochondrial precursor proteins pass.

Stop-Transfer Sequence

Hydrophobic stretch in a precursor protein that halts translocation and causes lateral release into a membrane bilayer.

Oxa1 Pathway

Mitochondrial route in which proteins first reach the matrix and are then inserted into the inner membrane by the Oxa1 translocase.

Hsc70

ATP-dependent chaperone that holds precursor proteins unfolded in the cytosol and helps pull them into the mitochondrial matrix.

Proton Electrochemical Gradient

Membrane potential and pH difference created by respiration; supplies energy that drives positively charged targeting sequences through Tim23.

Peroxisome

Single-membrane organelle that oxidizes fatty acids and detoxifies compounds, generating H₂O₂ that is broken down by catalase.

PTS1 Signal

Canonical peroxisomal targeting sequence (Ser-Lys-Leu) at the extreme C-terminus; not removed after import and allows folded proteins to enter.

Catalase

Peroxisomal enzyme that converts toxic hydrogen peroxide into water and oxygen.

Oxidase

Peroxisomal enzyme that transfers electrons from substrates to O₂, producing H₂O₂ as a by-product.

Nuclear Pore Complex (NPC)

Massive multiprotein gateway in the nuclear envelope that mediates bidirectional macromolecular transport.

Nucleoporins

≈30 different proteins, many containing FG-repeats, that assemble in multiple copies to form the NPC scaffold and selective barrier.

Nuclear Localization Signal (NLS)

Lys/Arg-rich peptide motif that is recognized by importin and directs proteins into the nucleus.

Importin

Cytosolic nuclear transport receptor that binds NLS-bearing cargo and ferries it through the NPC.

Exportin

Nuclear transport receptor that binds nuclear export signal (NES) cargo in the presence of Ran-GTP and carries it to the cytoplasm.

Ran GTPase Switch

Small GTP-binding protein whose GTP- or GDP-bound state controls the directionality of nuclear import and export.

GEF (Guanine nucleotide Exchange Factor)

Protein (e.g., RCC1) that replaces GDP with GTP on Ran, keeping Ran-GTP levels high inside the nucleus.

GAP (GTPase-Activating Protein)

Cytoplasmic factor that accelerates GTP hydrolysis on Ran, generating Ran-GDP outside the nucleus.

FG-Repeats

Phenylalanine-glycine motifs in nucleoporins that create a selective meshwork, transiently binding transport receptors.

Ran-Independent mRNA Export

Pathway in which processed mRNA binds NXF1/NXT1 and uses ATP-driven remodeling (Dbp5) at the NPC instead of the Ran gradient.

Intermembrane-Space-Targeting Sequence

Signal that directs proteins to the mitochondrial intermembrane space, often revealed after matrix-targeting sequence cleavage.

Chimeric Protein Experiment

Technique in which a targeting sequence is fused to an unrelated protein (e.g., DHFR) to test sufficiency and mechanism of organelle import.