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Lecture 9

Endoplasmic Reticulum

QuestionAnswer
endomembrane system ER, golgi, endosomes, lysosomes, nuclear envelope and all internal membranes
ER consists of a series of tubular membranes and flattened sacks called cisternae 2 types RER and SER
cisternal space space within the ER
RER 1. has ribosomes 2. protein synthesis 3. makes proteins for secretion and membrane bound proteins
polyribosome clustering of free ribosomes translating a single mRNA at same time
Ribosomes are attached to the cytosolic sife of the RER and contain rRNA (nucleic acid with a negative charge)
ergastoplasm in 1900s histologists referred to the dark basophilic regions of cell cytoplasm
Nissl body or substance dense clustering of RER and free ribosomes in neurons-dark staining regions of the cell body
Proteins made by the RER proteins for secretion enter into RER cisternal space for packaging into transition vesicles which bring them to the golgi for packaging int transport vesicles
cotranslational import the coupling of translation and import in ER for proteins that are made at and imported into RER
ERSS directed import at the onset of translation at the ribosome an ER signal sequence is present on the N-terminal that allows the ribosome to dock to the ER via a riboprotein complex SRP
SRP signal recognition particle contains 6 polypeptides and a 300 nucleotide rRNA that also temporarily blocks translation
How is ERSS cleaved off the N-terminal? by signal peptidase on the translocon
what happens to the newly made polypeptide in the RER cisternal space? polypeptide is modified in the RER lumen: 1. completion of folding 2. polypeptides may be glycoslylated 3. multimeric assembly of proteins 4. amino acid modification
Hsp 70 member BiP binding protein-binds to hydrophobic regions prevent the protein from aggregating with other hydrophobic proteins allowing it to fold properly
N-linked linked to the amide nitrogen of asparagine (ER)
O-linked linked to the hydroxyl group serine or threonine via galnac in golgi.
amino acid modification examples proline modified to hydroxyproline lysine modified t hydroxylysine
integral membrane proteins proteins with transmembrane domains are inserted into the ER membrane during assembly using stop transfer sequences or start transfer sequences and may also have an N-terminal ERSS
internal stop transfer sequence the stop transfer sequence is hydrophobic, located in the interior of polypeptide and stops the process of translocation at that site
internal start transfer sequence signal anchor sequence is hydrophobic and signals the SRP to dock with the translocon, the start transfer sequence enters the translocon then enters ER cisternal space.
multiple transmembrane domains altering patterns of start transfer and stop transfer signals allows for multiple transmembrane domains in a single polypeptide
smooth ER 1. no ribosomes 2. act as a series of partitions to isolate specific regions of the cytoplasm for specific functions;drug detoxification and carbohydrate metabolism 3. Ca storage 4. membrane biosynthesis 5. steroid hormone biosynthesis
cytochrome P-450 system adds a hydroxyl -OH to the drug and makes it more water soluble; aids in drug removal via the renal system
carbohydrate metabolism by glycogen breakdown and glucose formation in liver (glucose-6-phosphatase)
Ca 2+ storage in skeletal and cardiac muscle the SER is modified into the sarcoplasmic reticulum (modified SER) that stores calcium
membrane biosynthesis many enzymes associated with membrane biosynthesis are localized to the SER
steroid hormone biosynthesis enzymes associated with steriod hormone biosynthesis are localized to the SER endocrine cells that produce steroid hormones have lots of SER
Created by: aareynolds
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