Cell Biology- Chapter 15

Modes of Protein Transport

• Nuclear pores

• Translocators (organelles)

• Vesicles (everywhere else)

Protein Localization

• Determined by amino acid sequence

• Altering signal sequence changes localization

Nuclear Pores

Multi-protein complexes spanning nuclear envelope

Nuclear Pore Structure

• Cytosolic fibrils: bind nuclear import proteins

• Intrinsically disordered fibers: prevent random diffusion

Nuclear Import Process

• Nuclear import receptors recognize cargo in cytosol

• Receptors & cargo wiggle through nuclear pore fibers

• Inside nucleus, cargo is released

Nuclear Export

Similar to import but moves molecules (e.g., mRNA) out

Ran G Protein Cycle

• Ran-GTP: inside nucleus, releases cargo

• Ran-GDP: in cytosol, binds import receptors for return

Ran Regulation

• Ran-GEF (nucleus): promotes Ran-GTP

• Ran-GAP (cytosol): converts Ran-GTP to Ran-GDP

Mitochondrial & Chloroplast Protein Transport

• Proteins translated in cytosol

• Threaded into organelles via translocators

Peroxisome Protein Transport

Direct transport & ER shuttling by vesicles

Endoplasmic Reticulum (ER)

Largest membrane system in the cell

Ribosome Pool

Free vs. membrane-bound depends on the protein being synthesized

ER Signal Sequence

• Pauses translation

• Ribosome docks at ER membrane via signal-recognition particle (SRP)

Soluble ER Proteins

• Fully translocated into ER lumen

• Signal sequence is cleaved

Single-Pass Transmembrane Proteins

Stop-transfer sequence locks protein in membrane

Multi-Pass Transmembrane Proteins

Internal signal sequences & stop-transfer sequences create multiple domains

Vesicle Transport

Vesicles originate & fuse with membranes for transport

Clathrin-Coated Vesicles

Clathrin triskelions form basket-like structures

Cargo Selection for Vesicles

Clathrin attaches to cargo receptors via adaptins

ER Size Regulation

Based on demand for secretory & membrane proteins

Protein Glycosylation in ER

N-glycosylation: Sugar chains added to asparagine residues

ER Quality Control

• Misfolded proteins retained in ER

• Calnexin helps proper folding

ER Stress & Apoptosis

Accumulated misfolded proteins cause ER swelling → cell death

Golgi Apparatus Function

• Protein modification & sorting center

• Cis-face: receives vesicles

• Trans-face: ships vesicles

Exocytosis Types

• Constitutive: Continuous release

• Regulated: Signal-dependent release

Neurotransmitter Secretion

Voltage-gated calcium channels trigger vesicle fusion

Endocytosis Types

• Phagocytosis: Engulfs large particles (macrophages, neutrophils)

• Receptor-mediated endocytosis

• Pinocytosis: Exploited by viruses

Endocytosis Receptor Fates

• Recycling (returns to plasma membrane)

• Degradation (lysosome breakdown)

• Transcytosis (transport across the cell)

Lysosomal Digestion

• Acidic pH maintained by ATP-powered proton pump

• Metabolites exit lysosome via transporters

Lysosome Pathways

Phagocytosis, Endocytosis, Autophagy