Membrane-Enclosed Organelles and Protein Transport

Flashcards covering the mechanisms of protein import, organelle evolution, vesicle trafficking, and degradative pathways within eukaryotic cells.

Endomembrane System Evolution

The theory that internal membranes evolved from the invagination of the plasma membrane, creating a contiguous system from the nuclear envelope to the endoplasmic reticulum, Golgi, lysosomes, and plasma membrane.

Mitochondria and Chloroplast Addition

Evolved from independent aerobic prokaryotes that were engulfed by ancient anaerobic eukaryotic cells; they do not use vesicular transport to deliver materials.

Signal Sequences

Sections of amino acids, often $15-60$ aa in length, that are necessary and sufficient to direct a protein to its destination; properties like hydrophobicity or charge are more important than the exact sequence.

Nuclear Pore Complex

A large assembly consisting of ring subunits, cytosolic fibrils, and a nuclear basket that regulates transport between the cytosol and the nucleus.

Nuclear Localization Signal

A specific sequence on a protein that allows it to enter the nucleus; these signals and their receptors can be recycled for subsequent import or export.

Mitochondria/Chloroplast Import

Requires proteins to be in an unfolded state for continuous transport across both inner and outer membranes simultaneously.

Chaperones

Proteins that help pull polypeptides across membranes, assist in re-folding, and facilitate the assembly of multimeric proteins in the RER.

SRP (Signal Recognition Particle)

A particle that binds to the ER signal sequence on a growing polypeptide, slows translation, and docks the ribosome to an SRP receptor in the ER membrane.

Soluble ER Proteins

Proteins destined for secretion or the organelle lumen that possess an N-terminal signal sequence but no internal hydrophobic sequences.

Start/Stop Transfer Sequences

Hydrophobic sequences that determine protein orientation in the membrane; a stop sequence causes the protein to remain embedded while the remaining chain grows in the cytoplasm.

Clathrin

A protein coat that assembles into basket-like structures to shape the membrane into a bud and help capture cargo for transport.

Adaptins

Proteins that initiate vesicle budding by choosing specific cargo molecules and securing them to the protein coat.

KDEL

A specific amino acid sequence that acts as an ER retention signal, keeping proteins within the endoplasmic reticulum.

Rab Proteins

Small GTPases involved in vesicle docking that ensure the vesicle is delivered to the correct target membrane by interacting with tethering proteins.

v-SNARE and t-SNARE

Transmembrane proteins on the vesicle (v-SNARE) and target membrane (t-SNARE) that lock together to facilitate membrane fusion.

Disulfide Bonds ($S-S$)

Covalent bonds formed between sulfur atoms in the ER; they cannot form in the cytosol due to its reducing environment.

Glycosylation

The addition of pre-formed oligosaccharides to proteins in the ER, which protect the protein from degradation and guide it to its proper location.

Scramblase

An enzyme in the ER membrane that non-selectively distributes phospholipids across the membrane bilayer.

Flippase and Floppase

Selective enzymes that establish and maintain membrane asymmetry in the endomembrane system.

Unfolded Protein Response (UPR)

A cellular response triggered by the accumulation of misfolded proteins in the ER, which stimulates the production of more chaperones and transcription regulators.

Regulated Exocytosis

A secretory pathway where vesicles are stored for later use and only release their contents in response to specific signals, such as high $Ca^{+2}$ or low pH.

Pulse-Chase Experiment

A method using radioactive precursors to track the movement and flow of vesicles through the endomembrane system over time.

Phagocytosis

A non-specific endocytic pathway used by specialized cells like macrophages to ingest large particles or pathogens by forming pseudopods.

Pinocytosis

A non-specific 'cell drinking' mechanism where clathrin pits uptake extracellular fluid and solutes into endosomes.

Receptor-Mediated Endocytosis

A selective concentrating mechanism used to internalize specific molecules, such as cholesterol via LDL receptors.

Mannose-6-Phosphate

A sugar tag added in the ER that acts as a signal for the trans-Golgi receptor to deliver hydrolytic enzymes to the lysosome.

Autophagy

The degradation process used to digest foreign molecules and decrepit organelles by directing them to the lysosome via an autophagosome.