AC Lecture 33-35: Secretory pathways Flashcards
What is the function of the secretory pathway?
insertion of newly-made proteins in to membranes or secretion of newly made proteins from cells
What organelles are involved in the secretory pathway?
the ER (protein folding and quality control) the Golgi (sorting towards different parts of the cell) Lysosome (recycling cellular material that can be engulfed by membrane invagination)
Name the types of ER and their functions.
rough- binds to ribosomes so that the synthesis of proteins destined for membranes or secretion from the cell can have their synthesis coupled with translocation to the lumen (protein folding)
smooth- contains membrane-bound enzymes important for lipid synthesis and metabolism as well as detoxifying enzymes (e.g. cytochrome p450s)
How are newly made proteins targeted to the ER?
- Signal Recognition Particle (SRP) binds N-terminal signal peptide
while it is still being translated on the ribosome - translation is stalled
- complex binds to the ER membrane via an SRP receptor complex
- translation resumes and threads the building protein through the translocon’s aqueous channel into the lumen of the ER
5a(membrane bout proteins). channel opens sideways into the plane of the membrane so that the membrane-spanning domains of proteins become inserted into the membrane itself
Describe the environment of the ER versus the cytosol.
ER: oxidizing (helps to facilitate folding of proteins that must exist outside the cell (also an oxidizing environment)
cytosol: reducing
Describe the structure of the 14 residue carbohydrate that glycoslyates proteins entering the ER.
3 residues of glucose
9 residues of mannose
2 residues of N-acetyl glucosoamine (GlcNac)
*full structure is transferred from a dolichol anchor to the substrate protein
What is the role of the glucose residues added to proteins entering the ER?
important for protein folding- terminal two (of three) glucose are usually trimmed from core glycosyl unit leaving it in a monoglucosylated form which can bind calnexin (part of quality control) that retains proteins in the ER until they are folded properly
Describe the types of membrane proteins.
Type I: N-terminus in the lumen of the ER
Type II: C-terminus in the lumen of the ER
Topologically complex: many membrane spanning domains
What are membrane spanning domains and what is their function?
segments of membrane proteins comprised of hydrophobic amino acids that act as ‘start transfer’ and ‘stop transfer’ sequences
What are lectins and what is their function?
proteins that bind carbohydrates
(.g. Calnexin and Calrecticulin)
bind terminal glucose of the core glycosylation unit of newyl translocated proteins until their folding is completed in the presence of chaperones, peptidyl prolyl isomerase and protein disulphide isomerase
What are the phases of the ER quality control pathway?
1a. activation of the unfolded protein response (UPR) signaling pathway by aggregates
1b. expression of genes that encode ER-specific molecular chaperones and components of the ubiquitin/proteasome pathway
2. ER-associated degradation (ERAD)- luminal and membrane proteins are retrotranslocated from the ER to the cytosol for degradation by proteasome
Describe the movement of proteins through the Golgi apparatus.
- enter the cis Golgi in lipid vesicles from the ER membrane
- protein modification (trimmming of carbohydrates, phosphorylation, and sulfation) as the protein progresses through the cis Golgi
- protein sorting in the trans Golgi
What is the retrieval pathway?
returns resident ER proteins such as chaperones and other folding enzymes that inadvertently travel in vesicles to the Golgi
How do ER resident proteins accidentally sent to the Golge get back to the ER?
KDEL receptor in the golgi recognizes the KDEL sequence at the C-terminus of resident ER proteins and buds off in a vesicle back to the ER (retrieval pathway)
How do lysosomes get their enzymes?
via the secretory pathway (identified in the cis Golgi by an enzyme that phosphorylates a specific mannose residue of the core carbohydrate unit that was added in the ER; recognized in the trans-golgi by the mannose-6-phosphate receptor that helps sequester lysosomal enzymes into specific vesicles)
