MA3 Protein Synthesis and Secretion Flashcards
Polysome
a polyribosome; a cluster of ribosomes along a single strand of mRNA (amplification) having a spiral configuration for making proteins for intracellular use (can be free or membrane bound)
proteins made on free ribosomes don’t contain signal sequences for RER targeting
RER
A complex cytomembrane system with its outer surface studded with ribosomes
Interior region is called the intracisternal space, or lumen
Very active protein producing cells contain many parallel arrays or flattened saccules with greatly expanded (hypertrophied) lumens
Proteins destined to be secreted, plasma membrane proteins, and lysosomal proteins are translated on the RER
SER
An irregular network of membrane-bound tubules
No ribosomes on its surface
Appears as branching tubules or vesicles
SER arises from RER and may be continuous with the RER but is a separate organelle
Is prominent in cells making steroids, triglycerides, and cholesterol
Functions in steroid hormone synthesis, drug detoxification, and the release and recapture of calcium ions for muscle contraction
Transitional ER
the final cisternae of the RER which lacks ribosomes with small transport vesicles
Cis Golgi Network (CGN)
also called ERGIC; where transport vesicles enter from ER
Cis face of golgi
(entry, closest to the RER, convex, forming, immature); site of sugar modification on certain proteins
Medial compartment of Golgi
comprises a few cisternae lying between the cis and trans faces. this is the site of further sugar modificaiton
Trans face
(exit, concave, maturing); comprises the maturing cisternae, which are located at the side of the stack facing vacuoles and secretory granules, site of adding sialic acid and glactose (further glycosylation) and sulfation
Trans Golgi Network (TGN)
lies apart from the last cisterna at the trans face and is separated from the Golgi stack, sorting proteins for their final destinations
Vesicles
vesicles carry protein cargo between compartments. they pick up cargo by budding from the donor compartment and deliver cargo by fusion with the target compartment
there are 3 proteinaceous coats required for budding (COP I and II and clathrin)
other surface markers and receptors, along with microtubules and motor proteins (kinesin) are important for selecting cargo and moving vesicles between compartments
COP I
coatomer I; transports vesicles between golgi compartments (both anterograde and retrograde transport)
COP II
coatomer II; transports vesicles from the TER to the CGN (anterograde transport only)
Clathrin
transports vesicles from TGN to secretory granules or to lysosomes
Protein transport from RER to CGN
transport vesicles holding the newly synthesized protein bud off the TER via COP II coated vesicles and deliver proteins to the cis golgi network. From the CGN, proteins either move forward toward the cis Golgi face, or if they are RER-resident proteins they are returned in vesicles to the RER (COP I mediated)
Protein transport within the Golgi complex
budding off of COP I coated vesicles from one cisterna and fusion with the dilated rim of another
Protein processing in Golgi
may include the following, each occurs in a different cisternal compartment:
phosphorylation of mannose residues, removal of mannose, terminal glycosylation with sialic acid residues and galactose, sulfation and phosphorylation of amino acid residues
Sorting of regulatory secretory proteins in the TGN
regulatory secretory proteins are sorted from other proteins in the TGN and delivered via clathrin coated vesicles to secretory vesicles
Sorting of lysosomal proteins in the TGN
lysosomal proteins are tagged with M6P and are sorted into clathrin coated regions of the TGN that have receptors for mannose 6-phosphate (M6P) and are delivered to lysosomes (via late endosomes) via clathrin coated vesicles.
Sorting of plasma membrane proteins in the TGN
plasma membrane proteins are sorted into COP I coated regions of the TGN and delivered to the plasma membrane in COP I coated vesicles
Secretion
a process by which proteins synthesized in the RER are discharged outside the cell
constitutive and regulated secretion both occur via a specialized fusion event termed exocytosis (the process by which secretory vesicles or granules fuse with the plasma membrane to be discharged)
Constitutive secretion
the default pathway; is more or less continuous and needs no special stimulus; a good example is the secretion of antibodies from activated plasma cells. The rate of secretion can be up or down regulated. Vesicles (coated with COP I) undergoing constitutive secretion are not readily seen by EM because they rapidly fuse with the plasma membrane soon after budding from the TGN.
Not found in secretory granules (COP I vesicles)
Regulated secretion
“on demand” secretion and a stimulus (extracellular signal) is required; a good example is the secretion of hormones. Proteins that undergo regulated secretion are usually found in large clathrin-coated secretory granules, which may contain highly concentrated cargo proteins (thus appearing dark) and accumulate near the cell membrane where they will fuse
