Protein Transport Part 2: Mitochondrial and ER Import

Comprehensive vocabulary flashcards covering mitochondrial and endoplasmic reticulum protein transport, translocators, chaperones, and membrane protein topologies.

TOM complex (Translocase of the Outer Membrane)

A multi-protein complex in the outer mitochondrial membrane that contains receptor components for precursor proteins and forms translocation channels.

TIM complex (Translocase of the Inner Membrane)

A multi-protein complex in the inner mitochondrial membrane (such as TIM23) that mediates the translocation of proteins into the mitochondrial matrix or insertion into the inner membrane.

Mitochondrial DNA (mtDNA)

Circular mitochondrial genome encoding 37 genes (13 oxidative phosphorylation proteins, 2 rRNAs, and 22 tRNAs), with the remaining >1,000 to >1,500 mitochondrial proteins being encoded by the nuclear genome.

Molecular chaperones

Proteins that assist other proteins to properly fold and also keep polypeptides in a stable unfolded state in an ATP-dependent manner.

Cytosolic hsp70

A chaperone in the cytosol that maintains mitochondrial precursor proteins in an unfolded conformation and consumes ATP to help facilitate import through the TOM complex.

Mitochondrial hsp70

A matrix chaperone functioning as part of the import ATPase that binds the translocating polypeptide chain and undergoes energy-dependent conformational changes via ATP hydrolysis to pull the protein into the matrix.

Stop-transfer sequence

A hydrophobic stretch of amino acids that halts further translocation through the translocator channel, releasing the polypeptide into the bilayer to act as a membrane anchor.

Start-transfer sequence

An internal hydrophobic stretch of amino acids that is recognized by SRP to initiate translocation into the ER membrane; it is not cleaved and serves as a transmembrane domain in the mature protein.

Co-translational translocation

The primary import mechanism into the endoplasmic reticulum, wherein polypeptide translocation across the ER membrane occurs simultaneously with ongoing protein synthesis by a membrane-bound ribosome.

Post-translational protein import

A protein transport mechanism in which the polypeptide is fully synthesized and translated before import, characteristic of nuclear import and mostly mitochondrial import.

Rough ER

Regions of the endoplasmic reticulum that have actively translating ribosomes attached to their cytosolic surface.

Smooth ER

Regions of the endoplasmic reticulum that lack attached ribosomes and function in lipid synthesis and vesicular transport.

ER lumen

The continuous internal aqueous space enclosed by the membrane of the endoplasmic reticulum.

ER sheets and ER tubules

Distinct morphological structural domains of the endoplasmic reticulum visible by fluorescence microscopy, where sheets are continuous with the outer nuclear membrane.

Signal-recognition particle (SRP)

A cytosolic particle that binds both the hydrophobic ER signal sequence of a nascent peptide and the ribosome, temporarily pausing translation until targeted to the ER membrane.

SRP receptor targeting cycle

The process where ribosome-bound SRP docks to the SRP receptor on the ER membrane, releasing SRP for reuse and transferring the ribosome to the protein translocator so translation can resume.

Signal peptidase

An enzyme associated with the ER translocator that cleaves the N-terminal signal sequence from a translocating polypeptide, allowing the mature protein to be released into the lumen.

Soluble protein

A non-membrane protein that is completely translocated across the ER membrane into the ER lumen following cleavage of its signal sequence.

Flanking charge rule

The structural determinant of transmembrane orientation where the amino acid sequence flanking an internal start-transfer sequence that is more positively charged remains facing the cytosol.