Protein Sorting and the Cytoskeleton

Vocabulary flashcards covering protein sorting, signal sequences, the endomembrane system, post-translational processing, endocytosis/exocytosis pathways, and cytoskeletal structures from BIOL 1240.

Nuclear Pore Complex

A structure composed of approximately 30 proteins that spans the inner and outer nuclear membranes, allowing mRNA, tRNA, and rRNA to exit the nucleus and specific proteins to enter.

Nuclear Localization Signal (NLS)

A short amino acid sequence acting as a molecular address label that marks cytosolically synthesized proteins for transport into the nucleus.

Free Ribosomes

Ribosomes suspended in the cytosol that synthesize cytosolic soluble proteins, nuclear proteins, and proteins targeted to organelles outside the endomembrane system.

Rough ER-Associated Ribosomes

Ribosomes attached to the membrane of the rough endoplasmic reticulum that synthesize secreted proteins and proteins destined for organelles of the endomembrane system.

Signal Sequences

Distinct amino acid sequences within a protein that serve as address labels to target the protein to specific cellular compartments outside of the endomembrane system.

ER Signal Sequence

A peptide sequence synthesized at the start of translation that directs a growing polypeptide chain and its attached ribosome to the rough endoplasmic reticulum.

Signal Recognition Particle (SRP)

A protein-RNA complex that binds to the ER signal sequence on a nascent polypeptide, halts translation, and docks the ribosome onto an SRP receptor on the rough ER membrane.

Translocon

A protein channel embedded in the rough endoplasmic reticulum membrane through which a nascent polypeptide chain enters the ER lumen.

Secretory Pathway

The pathway in which proteins are synthesized into the rough ER, transported via vesicles to the cis Golgi apparatus, modified, packaged at the trans Golgi into vesicles, and delivered to the plasma membrane for release.

Vesicle

A small membrane-bound sac used to shuttle proteins, lipids, and other cargo between compartments of the endomembrane system or to the cell surface.

Golgi Apparatus

An endomembrane organelle that receives proteins at its cis face, modifies and sorts them based on molecular tags, and dispatches them in vesicles from its trans face.

Exocytosis

The secretion process wherein a transport vesicle fuses with the plasma membrane, releasing its soluble cargo into the extracellular fluid.

Phosphorylation

A post-translational modification where the addition of phosphate groups alters the shape and activity of a protein.

Glycosylation

A post-translational process of attaching carbohydrate chains to a protein, essential for protein targeting and cellular recognition.

Proteolysis

The enzymatic cleavage of a polypeptide chain during post-translational processing that allows resulting fragments to fold into functional shapes.

Phagocytosis

An endocytic process where a cell engulfs large particles or organisms into a phagosome, which subsequently fuses with a lysosome for digestion.

Autophagy

A recycling mechanism in which a damaged organelle is enclosed within a double membrane to form an autophagosome that fuses with a lysosome for degradation.

Receptor-Mediated Endocytosis

A selective uptake mechanism where specific extracellular molecules bind to cell-surface receptors, causing the membrane to pinch inward and form a vesicle targeted to an early endosome.

Actin Filaments (Microfilaments)

Cytoskeletal fibers made of two coiled strands of actin ($7\,nm$ in diameter) that maintain cell shape under tension, drive muscle contraction and cell crawling, and divide animal cells.

Intermediate Filaments

Fibrous cytoskeletal cables ($10\,nm$ in diameter) composed of proteins like keratins or lamins that resist tension and anchor the nucleus and other organelles.

Microtubules

Hollow cytoskeletal tubes ($25\,nm$ in diameter) formed from $\alpha$- and $\beta$-tubulin dimers that resist compression, form tracks for motor proteins, move chromosomes, and construct cilia and flagella.

Kinesin

A motor protein composed of a head, stalk, and tail that hydrolyzes ATP to walk along microtubule tracks toward the plus end during intracellular vesicle transport.

Dynein

A cytoskeletal motor protein that uses ATP energy to move cellular cargo along microtubule tracks toward the minus end.