Lecture 15- Vesicular Traffic, Secretion, and Endocytosis Part II

Receptor-Mediated Endocytosis

Uptake by a cell of certain extracellular macromolecules, bound to specific receptors in the plasma membrane, by pinching off the membrane to form small membrane-bound vesicles.; CLATHRIN DEPENDENT

Lysosome

Organelle for degradation of cellular waste; pH- <5.0

Phagocytosis

process in which extensions of cytoplasm surround and engulf large particles and take them into the cell; "cell eating"; coat independent process: fusion of membrane forms a phagosome that delivers invader ultimately to lysosome for degradation; CLATHRIN INDEPENDENT

Bulk Phase Endocytosis (pinocytosis)

AKA cell drinking. A mode of endocytosis in which small particles are brought into the cell, forming an invagination, and then suspended within small vesicles; retrieval of membrane components; CLATHRIN DEPENDENT

Trans-Golgi Network

•Distal sorting compartment
•Sorts proteins into five different types of vesicles for transport to the plasma membrane, endosomes, and lysosomes.

Mannose 6-Phosphate (M6P)

- a carbohydrate residue
- target soluble proteins to the lysosome/late endosome
- targeting requires a M6P receptor
- M6P is added to the lysosomal enzymes in the cis-Golgi

Clathrin

Protein forming vesicles from cell membrane.

Exocytosis (secretion)

Process of vesicle fusion with plasma membrane; constitutive and regulated pathways

Constitutive Secretion

Continuous release of proteins and lipids; provides cell with newly made lipids and proteins to resupply/replace worn out membrane components as well as increase SA for cell division

regulated exocytosis pathway

only occurs in cells specialized for secretion- such as hormone, digestive or neurotransmitter producing cells; package materials in vesicles and stored until a releasing signal is recognized

Dynamin

GTPase required for release of clathrin-coated vesicles from membrane; wraps around membrane stalk and GTP hydrolysis causes conformational change which severs membrane; pinches off the neck of a vesicle undergoing endocytosis

Endosomes

Vesicles transporting materials post-internalization.

Proteolytic Processing

Activation of proproteins into functional forms.

Apical Membrane

Surface of polarized cells facing lumen.

Basolateral Membrane

Surface of polarized cells facing underlying tissue.

Secretory Vesicles

Transport proteins to be secreted from cells.

clathrin-coated pits

aggregations of ligand-bound receptors on a cell membrane that pinches off and is internalized into the cell

Cytoskeleton

Network aiding in cell shape and movement.

Pseudopods

Cytoplasmic extensions for engulfing particles.

Ligand

Molecule that binds to a receptor.

Endocytic Pathway

•After internalization, vesicle-bound materials are transported in vesicles and tubules known as endosomes.
•Early endosomes are located near the periphery of the cell. It sorts materials and sends bound ligands to the late endosomes.
•Late endosomes are near the nucleus, also known as multivesicular bodies (MVBs). ; all transient in nature

Extracellular Matrix (ECM)

Network providing structural support to cells.

Insulin Release

Triggered by high blood sugar levels.

Late Endosomes

usually nearer to the nucleus; destination of lysosomal enzymes; ligands concentrated before transport to lysosomes for final processing; some receptors recycled to TGN; ubiquitin tagged receptors transported to lysosomes for final processing (degradation); pH- 5.5

Early Endosomes

located near the periphery of the cell; sorts materials and sends bound ligands to the late endosomes; recycling housekeeping receptors back to membrane; endosomal carrier vesicles formed; separates ligand from receptor; pH-6.5

Signal Sequence

Amino acid sequence directing protein localization.

Clathrin-Dependent Endocytosis

Endocytosis involving clathrin-coated pits.

Transmembrane Proteins

Proteins spanning the cell membrane.

Endocytic Route

Pathway for internalizing materials into cells.

H+-ATPase

Enzyme maintaining acidic pH in endosomes.

Ligand-Receptor Separation

Occurs in early endosomes due to pH change.

Lysosomal Enzymes

- dependent on low pH
several possible routes:

-maturation of late endosomes into lysosomes (changing protein complement)

-fusion of late endosomes w lysosomes (larger lysosome)

-transport from late endosomes to lysosomes in vesicles (vesicular transport)

Phagolysosome

Formed by fusion of lysosome and phagosome

Autophagy

break down damaged organelles; "self eating"; turnover of organelles during differentiation, digestion of organelles during starvation

House-Keeping Receptors

responsible for uptake of materials that will be used by the cell; recycled back to membrane and use pH to seperate ligand and receptor

Signaling Receptors

responsible for binding extracellular ligands that carry messages that change the activities of the cell; hormones, growth factors, etc

Clathrin-Coated Pits

aggregations of ligand-bound receptors on a cell membrane that pinches off and is internalized into the cell; vesicle is partially formed there; mediates more efficient transport into cell

LDL receptors

-mediate cholestrol uptake

-neural pH (cell surface): produces conformation that promotes binding between the LDL receptor and LDL particle

-acidic pH (in endosome): beta propeller domain histidine residues become protonated; positively charged propeller domain binds negatively charged ligand-binding domain residues-cause release of LDL particle

Transferrin

mediates iron uptake

Apotransferrin

Transferrin without bound iron.

Ferrotransferrin

Transferrin carrying iron in blood.

Ubiquitin Tagging

Marks proteins for degradation in lysosomes.

Multivesicular Endosome

Endosome type for degrading membrane proteins.

ESCRT Complexes

mediate the completion and pinching off of inwardly budding vesicles; recognize ubitquitin tag and assembles around Hrs protein

pH-Dependent Binding

Receptor conformational change based on pH.

Acid Hydrolases

Enzymes that degrade various macromolecules and that require an acidic pH to function properly. Acid hydrolases are found within the lysosomes of cells; proton pumps acidify environment

Lysosomal Digestion

Degradation of materials taken up by cells.

Endosomal Carrier Vesicles

Transport materials from early to late endosomes.

Lysosomal Environment

Acidic conditions for optimal enzyme activity.

Maturation of Late Endosomes

Process of transforming into functional lysosomes.

Receptor Down-Regulation

internalization of signaling receptors usually leads to degradation of the receptor after signaling cascade is initiated

Protonated Histidine Residues

Facilitate ligand release in acidic endosomes.

Clathrin/AP2-Coated Vesicles

Vesicles formed during receptor-mediated endocytosis.

constitutive secretory vesicles

- transport constitutively secreted proteins and plasma membrane proteins to the plasma membrane
- cargo proteins include ECM proteins ,blood proteins, and immunoglobulins

regulated secretory vesicles

- store and process secreted proteins until signaled to fuse with the plasma membrane to secrete the proteins
- cargo proteins include digestive enzymes and peptide hormones

AP complex proteins

provide specificity for recruitment of clathrin-coated vesicle cargo

Clathrin protein shape

spherical formation leads to formation of vesicle; scaffolding

M6P pH

cytoplasm: basic pH (favor M6P binding)
lysosome: acidic pH (separate protein from receptor)

proproteins

•Matured into final form after leaving the trans-Golgi
•Include soluble lysosomal enzymes; many membrane proteins, such as influenza hemagglutinin (HA); and secreted proteins such as serum albumin, insulin, glucagon, and the yeast α mating factor. PRO=precursor to functional protein

functions of lysosomes

degradation of ligands and dissolved macromolecules taken up by endocytosis; digestion of solid materials brought into cells by phagocytosis

-digestion of cellular organelles by autophagy
-surrounded by membrane of ER

Mitaphagy

organelles which are damaged will be removed

RME receptors

some types cluster in clathrin coated pits by cytoplasmic association w AP2in absence of ligand; other diffuse freely in plasma membrane until ligand induced conformational change associates them w AP2; LDL and transferrin are examples

Model of low density lipoprotein (LDL)

-shell composed of apolioprotein and phospholipid monolayer
-hydrophobic core composed of mostly cholesteryl esters/trigylcerides
-well-defined lipoprotein complexes

LDL particle

contains only a single molecule of one type of apolipoprotein (ApoB) wrapped around the outside of the particle; transports large amounts of cholestrol throughout body

transferrin cycle

delivers iron to cells without dissociation of the transferrin- transferrin receptor complex in endosomes; iron is cargo; transport iron inside proteins; change in pH doesnt interfere w rxn between ferrotransferrin and receptor

multivesicular endosome formation mechanism

-ubiquitinylated Hrs protein on endosomal membrane directs loading of ubiquitinylated membrane cargo proteins into endosome vesicle buds

-recruits and assembles cytosolic ESCRT protein complexes on endosome membrane