Cell Biology 3 Flashcards
Endocytosis-5 pathways
macropinocytosis, clarthin-mediated endocytosis, non-coated-mediated endocytosis, caveolae-mediated endocytosis, phagocytosis
–ATP and Ca2+ are required for these processes
Macropinocytosis
(occurs in thyroid cells as they take up thyroglobulin and dendritic cells for immune surveillance)
- Actin-based process
- Nonspecific ingestion of fluid and solutes
- Can be triggered by bacteria
Clarthrin-mediated endocytosis
- Occurs at clathrin-coated pits
- Dynamin (GTPase) is required to pinch off the vesicle
- Can be mediated by receptors – a process to endocytose specific molecules
a) Cholesterol, in form of the low-density lipoprotein (LDL), is taken up by cells in this process
b) Protein hormones - Coated pits
a) Cargo receptors are located here (capping)
b) Adaptin associates with the intracellular portion of the cargo receptor
c) Coated vesicles form: become uncoated quickly after they are formed
d) Translation to clinical relevance – Atherosclerosis occurs when there is a decrease or defect in LDL receptors - Recycling of synaptic vesicles
Nonclathrin/noncaveolar
Cholera & Shiga toxins
not using a coating protein
Phagocytosis
- Ingestion of large particles (cell eating) that is generally mediated by receptors (zipper-mediated)
- Dependent on actin, a cytoskeletal element; does not require clathrin
- Phagosomes
- Fuse with lysosomes
Secretion-3 pathways
-Exocytosis, Porocytosis, Exosomes & exosomes-like vesicles
Exocytosis
(bulk secretion)
- Secretion of cellular synthetic products
a) Proteins and enzymes
b) Hormones
c) Neurotransmitters - Secretory vesicles, formed by the Golgi complex, fuse with the plasma membrane. Intracellular trafficking of vesicles is orchestrated by COPs (COat Proteins).
- Two pathways
a) Constitutive – continuous secretory process; secretory product is not stored in secretory granules
b) Regulated – secretory product is stored in secretory vesicles until a signal causes the product to be secreted. - Requires calcium and ATP
Porocytosis
(quantal secretion) – quantal release of neurotransmitters
–able to release a small amount whereas exocytosis releases everything
Exosomes & exosome-like vesicles
- Secretory products are released into the extracellular environment limited by a membrane
- One function is to discard unneeded membrane proteins (e.g., reticulocytes discard the transferrin receptor in this manner)
- Some tumors release exosomes and thus may represent a biomarker.
Protein synthesis
The role of ribosomes (nonmembrane-bound organelles) and rough endoplasmic reticulum
Rough ER
- cisternae are studded with polyribosomes
- Basophilic
- Continuous with outer membrane of nuclear envelope
Function of rough ER
a. Synthesis of proteins for secretion, membrane proteins, and lysosomal proteins
b. Synthesis of enzymes associated with the sER
c. Modification of proteins (ex. addition of carbohydrate moieties»_space; glycoproteins and to assist with protein folding)
- -Rough ER is a prominent feature of protein-secreting cells, such as pancreatic acinar cells: proteins to be secreted are synthesized by rER
Free polyribosomes (polysomes)
i. Messenger RNA + ribosomes (pearls on a necklace appearance)
ii. Protein synthesis for use within the cell, generally
a.Hemoglobin in immature erythrocytes
b. Many mitochondrial proteins
c. Proteins in peroxisomes
Note: Peroxisomal and mitochondrial proteins are synthesized in the cytoplasm and then imported into the organelle by translocator proteins
Endoplasmic reticulum stress (ER Stress)
A. Definition – accumulation of unfolded/misfolded proteins in the ER cisterna.
B. Unfolded protein response
1. Chaperone synthesis increases in attempt to repair the unfolded/misfolded proteins
2. Decreased synthesis of the proteins
3. Misfolded proteins are exported from the ER to the cytosol where they are tagged for degradation by proteasomes.
4. Caspases activated → apoptosis
Post-translational targeting of proteins
A. Ribosomes serve as a common manufacturing site for proteins that must be delivered to various cellular compartments (e.g., mitochondrion, peroxisome, plasmalemma, secretory pathway, nucleus).
B. Targeting signals (molecular zip codes) are sequences of amino acids in the protein
C. Targeting signals direct proteins to their target compartments by binding to receptors that are specific for the organelles. Escort factors may deliver the protein to the target organelle.
D. In some cases, the delivered protein will cross the membrane of the organelle. This is accomplished by transmembrane proteins called translocons and the process is termed translocation.
