Histo: Protein Degradation and Protein Synthesis Flashcards
What are endosomes?
Sort proteins that have been internalized by endocytotic processes
The fate of the internalized ligand-receptor complex depends on…
The sorting and recycling ability of the early endosome.
What are lysosomes?
Digestive organelles rich in hydrolytic enzymes that degrades macromolecules from endocytosis and the cell itself
Digesting cytoplasmic components is called…
Autophagy
What are examples of hydrolytic enzymes?
Proteases, nucleases, glycosidases, lipases, phospholipases
Where are lysosomes synthesized and sorted?
They are synthesized in the rER and sorted in the Golgi apparatus based on binding ability to M-6-P receptors
(T/F) The membrane of lysosomes is not resistant to hydrolytic digestion occurring in their lumen.
False - It is
The lysosomal membrane contains…
Unusual phospholipid structure, cholesterol, lysobisphosphatidic acid, structural membrane proteins (lamps, lgps, limps), proton pumps, transport proteins (transport products of digestion to the cytoplasm)
Characteristics of lysosomal structural proteins:
~50% of total membrane proteins, highly glycosylated on luminal surface, sugar molecules cover almost entire luminal surface to protect from digestion by hydrolytic acids
What is the probable role of lysobisphosphatidic acids?
They may help with restricting the activity of hydrolytic enzymes directed against the membrane
What is chloroquinone?
Agent used in treatment and prevention of malaria that accumulates in lysosomes and raises the pH of the lysosomal content, inactivating many lysosomal enzymes. It concentrates in acidic food vacuole of malaria parasite, interfering with digestive processes and eventually killing it.
What is the constitutive secretory pathway for lysosomal membrane proteins?
Limps exit Golgi apparatus in coated vesicles, are delivered to cell surface, endocytosed, via early/late endosomes, reach lysosomes.
What is the Golgi-derived coated vesicle secretory pathway from lysosomal membrane proteins?
Limps exit Golgi apparatus in clathrin-coated vesicles, transport vesicles fuse with late endosomes through interactions with v-SNARE and t-SNARE
3 pathways for delivering material for intracellular digestion in lysosomes
1) Extracellular large particles: phagocytosis, phagosome matures into lysosome
2) Extracellular small particles: pinocytosis and receptor-mediated endocytosis, early–>late endosomes–>lysosomes
3) Intracellular particles: isolated from cytoplasmic matrix by ER membranes, transported to lysosomes
What is an example of cells that release lysosomal enzymes directly into the extracellular space to digest components of the ECM?
Osteoclasts involved in bone resorption, neutrophils involved in acute inflammation
What is a residual body?
Debris-filled vacuole that results from hydrolytic breakdown of contents of lysosomes. May remain in cell for its entire life. Ex. age pigment/lipofuscin granules in neurons
What leads to lysosomal storage diseases?
Absence of certain lysosomal enzymes that causes pathologic accumulation of undigested substrate in residual bodies
(T/F) Digested components of organelles are recycled and reused for normal cell growth and development.
True
Autophagy plays an essential role in:
Nutrient starvation, cellular differentiation, cell death, aging, hypoxia, high temperature
The presence of adequate nutrients and growth factors stimulates enzymatic activity of…
mTOR, a serine/threonine kinase
High mTOR activity exhibits…
An inhibitory effect on autophagia
Lack of mTOR activity causes activation of…
Atg genes, which results in formation of Atg1 protein-kinase autophagy-regulatory complex that initiates process of autophagy.
3 pathways of autophagy
1) Macroautophagy
2) Microautophagy
3) Chaperone-mediated autophagy
Describe macroautophagy.
Nonspecific process in which portion of cytoplasm or entire organelle is surrounded by a double ER membrane (isolation membrane) to form vacuole called autophagosome where digestion occurs. The isolation membrane (inner) disintegrates within hydrolytic compartment of the new lysosome.
(T/F) An autophagosome matures into a lysosome.
True - after targeted delivery of lysosomal enzymes
Macroautophagy occurs in…
The liver during first stages of starvation
Describe microautophagy.
Nonspecific process in which cytoplasmic proteins are degraded in lysosomes by a slow, continuous process under normal physiologic conditions. These proteins are internalized into the lysosome by invagination of the lysosomal membrane.
Describe chaperone-mediated autophagy.
Only selective process of protein degradation which requires assistance from specific cytosolic chaperones such as hsc73 which binds to the protein and assists in transport to lysosome lumen. Process is activated during nutrient deprivation and requires presence of targeting signals on degrading proteins and a specific receptor on the lysosomal membrane.
Chaperone-mediated autophagy is responsible for degradation of ~ what percent of cytoplasmic proteins in organs such as the liver and kidney?
30%
Other than lysosomes, what is another way cells can destroy proteins?
Proteosomes
What are proteosomes?
Large cytoplasmic/nuclear protein complexes that destroy specific proteins.
Proteosome-mediated degradation is used by cells to destroy what kinds of proteins?
Abnormal proteins that are misfolded, denatured, or contain abnormal amino acids. Also short-lived regulatory proteins that need to be rapidly inactivated and degraded (ex. mitotic cyclins, transcription factors, tumor suppressors, tumor promoters).
What do mitotic cyclins do?
Regulate cell-cycle progression
Proteins destined for proteosome-mediated degradation need to be recognized and specifically tagged by…
The polyubiquitin chain
2 steps of degradation of a protein in the proteosome-mediated pathway
1) Polyubiquitination: proteins tagged by ubiquitin molecules (forming a chain)
2) Degradation of the tagged protein by the 26S proteosome complex
Polyubiquitination is catalyzed by
3 ubiquitin ligases called ubiquitin-activating enzymes E1, E2, and E3
Structure of a proteosome
Hollow cylinder containing a 20S CP that facilitates multicatalytic protease activity in which polyubiquitinated proteins are degraded into small polypeptides and amino acids. 19S RPs on both ends - one forms lid and recognizes polyubiquitin tags, unfolds protein, and regulates entry. Other releases short peptides and amino acids after degradation.
Free ubiquitin molecules in proteosome are released by…
DUB enzymes and recycled.
What are the 2 groups of pathologic conditions associated with the malfunction of proteasome-mediated degradation?
1) Diseases caused by mutations in the system of ubiquitin-activating enzymes (ex. Angelman syndrome, Alzheimer’s disease)
2) Diseases caused by accelerated degradation of proteins by overexpressed proteins involved in this system (ex. infections with HIV)
Ergastoplasm
Portion of cytoplasm with lots of RNA that stains with basic dyes
The ergastoplasm in secretory cells such as pancreatic acinar cells is…
rER
With TEM, the rER appears as…
Series of interconnected, membrane-limited, flattened sacs called cisternae with particles studding exterior surface (ribosomes)
Ribosomes are attached to the rER membrane by…
Ribosomal docking proteins
Ribosome structure
Small and large subunit, each with rRNA of different length, also numerous proteins
(T/F) rER is often continuous with outer membrane of the nuclear envelope.
True
What are polysomes?
Polyribosomes - short spiral arrays formed by groups of ribosomes attached to thread of mRNA
Post-transcriptional modifications of the pre-mRNA molecule include:
RNA cleavage, excision of introns, rejoining of exons, and capping by addition of poly(A) tracks at the 3’ end and a methylguanosine cap at the 5’ end
A typical single cytoplasmic mRNA binds to many ribosomes spaced as close as 80 nucleotides apart, forming a…
Polyribosome complex/polysome
(T/F) A polysome can translate a single mRNA mlcl and simultaneously produce many copies of a particular protein
True
(T/F) Free ribosomes are structurally and functionally identical to polysomes of the rER.
True
What is the difference between the structure of prokaryotic (bacterial) and eukaryotic ribosomes?
Antibiotics can bind to bacterial ribosomes and inhibit protein synthesis - doesn’t harm infected individual.
Examples of antibiotics that inhibit protein synthesis by binding to bacterial ribosomes
Aminoglycosides (streptomycin), macrolides (erythromycin), lincosamides (clindamycin), tetracyclines, chloramphenicol
Signal sequences for sorting proteins are found where?
In the sequence of the first group of 15 - 60 amino acids on the amino-terminus of a newly synthesized protein
Signal sequences interact with…
SRPs, which arrest further growth of the polypeptide chain
After interacting with signal sequence, SRP binds to what?
A docking protein on the cytoplasmic surface of the rER. This aligns the ribosome with the translocator (integral membrane protein of rER), causing dissociation of the SRP-docking protein complex away from the ribosome and rER membrane. Ribosome resumes protein synthesis.
