Intracellular Transport Lecture Notes

Vocabulary flashcards covering protein localization to organelles, ER function, and associated mechanisms.

SKL Sequence

A frequent motif (Ser-Lys-Leu) in peroxisomal-targeting sequences (PTSs) found at the C-terminus of polypeptides.

Peroxisomal-Targeting Sequences (PTSs)

Specific amino acid sequences, such as the SKL sequence, that localize a protein to the peroxisome.

Pex5

A PTS receptor found in the cytosol that escorts polypeptides containing an SKL sequence to the peroxisome.

Zellweger syndrome

A rare genetic condition caused by a mutation in the Pex5 gene, leading to the peroxisome's inability to break down lipids, which accumulate to lethal levels.

Matrix-targeting sequences (MTS)

Sequences, often nonpolar and positively charged (basic) amino acids at the N-terminus, that target unfolded proteins to plastids.

Chaperone proteins

Proteins (e.g., Hsp70) required to keep unfolded polypeptides stable while they travel to organelles like plastids.

Tomm proteins (Translocase of Outer Mitochondrial Membrane)

Proteins that bind and translocate polypeptides across the outer plastid/mitochondrial membrane to Timm proteins.

Timm proteins (Translocase of Inner Mitochondrial Membrane)

Proteins that finish translocating polypeptides into the matrix space of plastids/mitochondria.

Importin

A cytosolic carrier protein that binds nuclear-localization signals and interacts with FG-repeats of the nuclear pore complex to facilitate nuclear entry.

Ran-GTP

A protein that causes importin to release its polypeptide cargo once inside the nucleus and is required for proteins to exit the nucleus by binding with exportin.

GTPase Activating Protein (GAP)

An enzyme that hydrolyzes GTP, causing importin to dissociate (after nuclear import) or causing polypeptide, exportin, and Ran-GTP to dissociate (after nuclear export).

Nuclear-export signal

An amino acid sequence required for proteins to bind with Ran-GTP and exportin to exit the nucleus.

Exportin

A protein that binds with Ran-GTP and proteins containing a nuclear-export signal to facilitate their exit from the nucleus.

Cotranslational Translocation

The process where polypeptides with signal sequences (N-terminus or internal) are recognized by SRP before translation completes and are guided to the ER for entry.

Signal Recognition Particle (SRP)

A ribonucleoprotein that recognizes signal sequences in the cytosol before translation completes and binds to the signal sequence and the large subunit of the ribosome.

SRP-receptor protein

A protein embedded in the ER membrane that forms a complex with the SRP-ribosome-polypeptide complex during cotranslational translocation.

Translocon

A protein channel in the ER membrane through which polypeptides pass into the ER lumen during cotranslational translocation.

Get3 ATPase (Guided Entry of Tail-anchored protein ATPase)

An enzyme that binds C-terminus signal sequences of fully translated proteins and facilitates their transfer into the ER membrane by hydrolyzing ATP.

Microsomes

Isolated and purified fragments of the endoplasmic reticulum used in experiments to study ER function.

Stop-Anchor (SA) sequence

A sequence in single-pass and multi-pass proteins containing a hydrophobic region followed by basic amino acids that determines protein orientation in the membrane and cannot be removed.

Signal peptidase

An enzyme in the ER that cleaves N-terminal signal sequences from polypeptides after they have entered the ER lumen.

Stop transfer anchor (STA)

An internal hydrophobic sequence that causes a polypeptide to stop being extruded into the ER lumen and instead become embedded in the ER membrane.

GPI transamidase

An enzyme that cleaves a polypeptide from a Stop-transfer anchor (STA) and covalently attaches it to a GPI anchor if a transamidase site is present.

GPI anchor (Glycosylphosphatidylinositol anchor)

A phosphoglyceride that can be covalently attached to a polypeptide by GPI transamidase, forming a lipoprotein.

Lipoprotein

A final structure formed when a polypeptide is covalently attached to a GPI anchor, which is a phosphoglyceride.

Signal peptide peptidase (SPP)

An ER enzyme that normally breaks down N-terminus signal sequences that have been cleaved from ER localized proteins.