Secretory Pathways- Lectures 33-35 Flashcards
What organelles are involved in the secretory pathway?
Endoplasmic Reticulum (protein folding and quality control) Golgi Apparatus (sorting) Lysosome (recycling cellular materials that can be engulfed by membrane invagination) Lipid Vesicles (transportation)
What are the two types of ER?
rough (ribosomal) and smooth
How are new proteins targeted in the ER?
N-terminal signal peptide (8-20 residues and enriched hydrophobic amino acids) targets newly made proteins
internal targeting sequences may not be cleaved after import so signal peptide binds to Signal Recognition Particle (SRP) - a ribonucleoprotein complex that attaches to newly synthesized proteins while they are still being translated
What happens when SRP binds a peptide?
translation is stalled and the complex of translating ribosome and SRP bind to the ER membrane (via SRP receptor complex) –> translation is resumed when positioned next to the translocon (adjacent to the SRP receptor site) where it can continue translation through the aqueous channel and into the lumen of the ER (or channel can open sideways into the plane of the membrane to create membrane spanning domains)
The ER is an ____ environment, where as the cytosol is a ____ environment.
oxidizing
reducing
Why is it important that the ER is an oxidizing environment?
because it helps to facilitate folding of proteins that must exist outside the cell (also an oxidizing environment)
Proteins entering the ER are glycosylated on _____ via a ____ residue carbohydrate structure that includes _____.
asparagine (N-linked glycosylation)
14
3 residues of glucose, 9 residues of mannosse, and 2 residues of N-acetyl glucoamine (GlcNac)
When are proteins entering the ER glycosylated on asparagine?
when it is part of the sequence Asn-X-Ser or Asn-X-Thr
Describe the sequence of events that is required for protein to be released from the ER.
the terminal two glucose residues are trimmed from the core glycosyl unit (resulting in monoglucosylate form) –> binding to calnexin (part of quality control apparatus) until fully folded –>
IF properly folded, last glucose is cleaved and protein can be released from the ER and onto the next stage of the pathway
IF not properly folded, glucose is added back until it is properly folded
What are the two kinds of membrane proteins?
Type I (N-terminus in the lumen of the ER) Type II (C-terminus in the lumen of the ER) Topology Complex (multiple membrane-spanning domains)
_____ are lectins- proteins that bind carbohydrates.
Calnexin and Calrecticulin
Describe the ER quality control pathway.
capacity of chaperone apparatus is exceeded by unfolded protein –>
- activation of signaling pathway unfolded protein response (UPR) –> expression of genes that encode ER-specific molecular chaperones and components of ubiquitin/proteasome pathway
- ER-associated degradation (ERAD)- luminal and membrane proteins are retrotranslocated from the ER to the cytosol for degradation by proteasome
Once proteins fold properly in the ER, they begin their journey via _____ to the _____.
lipid vesicles
the Golgi apparatus
Describe the structure of the Golgi apparatus.
stack of flat, membranous disks with directionality (cis to trans end movement of proteins)
What occurs in the Golgi stacks?
post-transcriptional modifications (eg. trimming of carbohydrates, phosphorylation, and sulfation)
What occurs in the trans Golgi network?
proteins sorted from each other and packaged into transport vesicles
What does the operation of the retrieval pathway require?
KDEL receptors that recognize a KDEL sequence at the C-terminus of resident ER proteins that are accidentally sent to the Golgi complex in vesicles
What are the key characteristics of a lysosome?
built for recycling/digesting anything
full of acid hydrolases
consistently have a pH of 5 (maintained by proton pumps)
Describe the travel of acid-hydrolases to the lysosome.
- acid hydrolases in cis golgi have carbohydrates that are recognized by phototransferase –> phosphorylation manose @ 6 position
- protein in membrane of trans golgi (pH of 6) that has a receptor allowing for specific vesicles to emerge that just have this receptor and phosphorylated lysosomal protein
- vesicles fuse with endosome (pH 5.5) where the pH difference causes reduction in affinity of mono 6 phosphate receptor for enzyme –> proteins in the endosome can then transfer the two individually to their respective destinations
Provide examples of lysosomal storage diseases.
Gaucher’s Disease (glucocerebrosidase mutation resulting in improper processing of sphingolipids- can be treated with transfusions of mono6phosphate which can enter due to the few receptors that accidentally end up in the plasma membrane during the return from lysosome)
Fabry’s Disease (mutation in alpha-galactosidase A- can be treated with DGJ replacement therapy)
What is the difference between constitutively secreted proteins and regulated secretion?
Constitutive (eg. extra-cellular matrix proteins)- travel directly to the plasma membrane and fuse
Regulated (eg. hormones, enzymes, neurotransmitters, and milk proteins)- require a specific signal before they can fuse with the plasma membrane
What is the difference between proteins targeted to the basolateral membrane and the apical membrane?
basolateral- have distinct target sequences
apical- get there by several different mechanisms (including specific targeting sequences and glycosylphosphatidyl inositol anchors that are attached in the ER) via lipid rafts
What are the main types of endocytosis?
phagocytosis/ pinocytosis (differentiated by size)
receptor mediated endocytosis
autophagy
Describe phagocytosis.
macrophages (professional scavenging cells) invaginate very large areas of the plasma membrane to encompass pathogenic bacteria
normally for particles up to about 0.5u in diameter
