Endocytosis and Vesicular Transport

• Vesicular transport moves materials between the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and the cell exterior.

Endocytosis

• Endocytosis is the process by which cells take up materials from the extracellular space by invaginating the plasma membrane.
• This process encloses the extracellular fluid.
• There are three types of endocytosis:
  • Phagocytosis, where cells engulf large particles like bacteria or cell debris.
  • Pinocytosis, where cells take up extracellular fluid.
  • Receptor-mediated endocytosis, where specific receptors on the cell surface bind to cargo molecules, triggering endocytosis.

Vesicular Trafficking

• Vesicular trafficking enables proteins and vesicles to reach their destinations.
• This process involves budding from a donor compartment and fusion with a target compartment facilitated by coat proteins such as clathrin, COPI, and COPII.

Coat Proteins

• Different coat proteins serve distinct roles:
  • Clathrin is associated with the plasma membrane, late endosomes, and the trans-Golgi network.
  • COPI functions between the Golgi cisternae and from the cis-Golgi to the ER.
  • COPII transports from the ER to the cis-Golgi.
  • Retromer is involved with the late endosome.
• Clathrin-coated vesicles consist of clathrin, which has three large and three small polypeptide chains.
• Adaptor proteins (APs) are required to bind the clathrin coat to the membrane to trap various transmembrane proteins.

AP Binds and Dynamin

• AP binds to phosphoinositides, which help to regulate membrane budding and vesicle formation.
• Dynamin is a soluble cytoplasmic protein that assembles as a ring around the neck of each bud and pinches off the membrane to release clathrin-coated vesicles.

SNARE Proteins

• Transport vesicles are highly selective and display surface markers that identify them according to their origin and type of cargo.
• SNARE proteins have a central role both in providing specificity and in catalyzing the fusion of vesicles with the target membrane.
• v-SNAREs are incorporated into the membranes of transport vesicles during budding, while t-SNAREs are located in the membranes of target compartments.
• When a v-SNARE interacts with a t-SNARE, the helical domains wrap to form stable trans-SNARE complexes, which lock the two membranes together.
• NSF (N-ethylmaleimide-sensitive factor) is an ATPase that dissociates SNARE pairs.

Rab Proteins

• Rab proteins direct vesicles to specific spots on the target membrane.
• Rab proteins on the vesicle are recognized by tethering proteins on the target membrane, facilitating docking.

ER and Golgi Apparatus

• The ER and Golgi apparatus work together in protein processing and sorting.
• Transport from the ER goes through the Golgi.
• The Golgi apparatus is a sorting and dispatching station for the products of the ER.
• The Golgi apparatus synthesizes carbohydrates and glycosaminoglycans.
• Proteins leave the ER in COPII-coated vesicles.
• Cargo proteins display exit signals, recognized by receptor proteins.
• Proteins must be properly folded or completely assembled to be transported from the ER to the Golgi.
• Misfolded or incompletely assembled proteins are retained in the ER and bound by chaperone proteins.
• Specific sorting signals direct proteins to specific transport vesicles, such as the KDEL sequence for ER-resident proteins.
• Membrane proteins with a KKXX sequence are directed into COPI vesicles.

Lysosomes

• Lysosomes contain approximately 40 types of hydrolytic enzymes, including proteases, nucleases, glycosidases, lipases, phospholipases, phosphatases, and sulfatases.
• Lysosomes are a major site for degradation and recycling of macromolecules.
• Lysosomes maintain an acidic lumen.
• Mannose 6-phosphate receptors recognize lysosomal proteins in the trans-Golgi network.
• Inclusion-cell disease results in a deficiency of hydrolytic enzymes in lysosomes, leading to accumulation of undigested substrates and coarse facial features/skeletal abnormalities.

Cytoskeleton

• The cytoskeleton gives the cell structure and shape, helps to move vesicles and chromosomes.
• Cytoplasm is crowded.

Microtubules and Kinesin

• Microtubules are responsible for intracellular transport and positioning of membrane vesicles and organelles.
• Kinesin walks towards the plus ends of microtubules.
• Kinesin can be used for anterograde axonal transport.

Dyneins

• Dyneins are minus end-directed microtubule motors involved in organelle/mRNA transport.
• Dyneins are also involved in positioning the centrosomes/nucleus as well as construction of the microtubule spindle.

Colchicine

• Colchicine can kill cells.
• Colchicine is a microtubule depolymerization inducer.