Christine's Report

7/28/2019 Christine's Report

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LETS LEARN MORE ABOUT ENDOPLASMIC

RETICULUM...


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THREE WAYS IN WHICH SINGLE PASS

TRANSMEMBRANE PROTEINS CAN BE INSERTED

INTO THE ER:

(1) Simplest case: an amino-

terminal signal peptide initiates

translocation, just as for a solubleprotein., but an additional

hydrophobic segment in the

polypeptide chain stops thetransfer process before the entire

polypeptide chain is translocated.


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Stop transfer peptide- anchors the

protein in the membrane after theER signal peptide is released from

the translocator and is cleaved off.Forms a single alpha-helical

membrane-spanning segment, with

the amino temrinus of the protein onthe luminal side of the membrane

and the carboxyl terminus on the

cytosolic side.


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(2,3) In the other 2 cases:

The signal peptide is internal, rather

than at the terminal end of the protein.


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MULTIPASS TRANSMEMBRANE PROTEINS:

the polypeptide chain passes back and forth

repeatedly across the lipid bilayer.


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TRANSLOCATED POLYPEPTIDE CHAINS FOLD

AND ASSEMBLE IN THE LUMEN OF THE ROUGH

ER:

ER Resident proteins

Contain an ER retention signal of four

amino acids at their carboxyl terminus that

is responsible for retaining the protein in

the ER.

Some of these proteins function as

catalysts that help many proteins that are

translocated into the ER to fold and

assemble correctly.


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EXAMPLE OF ER RESIDENT PROTEINS:

(1) Protein disulfide isomerase

Catalyzes the oxidation of the free

sulfhydryl (SH) groups to form

disulfide bonds.


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EXAMPLES...

(2) Binding protein (BiP)

Chaperone protein that is structurally

related to hsp70 proteins and, likethem, recognizes incorrectly folded

proteins, as well as protein subunits

that have not yet assembled into theirfinal oligomeric complexes.


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10/18

SOME MEMBRANE PROTEINS EXCHANGE A

CARBOXYL-TERMINAL TRANSMEMBRANE TAIL

FOR A COVALENTLY ATTACHEDGLYCOSYLPHOSPHATIDYLINOSITOL (GPI) AFTER

ENTRY INTO THE ER

Glycosylphosphatidylinositol (GPI)

Contains two fatty acids.


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MOST MEMBRANE LIPID BILAYERS ARE

ASSEMBLED IN THE ER

The ER membrane produces nearly all of the

lipids required for the elaboration of new cell

membranes, including both phospholipids

and cholesterol.The major phospholipid made is

phosphatidylcholine (or lecithin), which can be

formed in three steps from choline, two fattyacids, and glycerol phosphate.


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2 OTHER MAJOR MEMBRANE PHOSPHOLIPIDS

Phosphatidylethanolamine (PE) and

Phosphatidylserine (PS) as well as the

minor phospholipid Phosphatidylinositol(PI), are synthesized in the same

manner as that of the lecithin.


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Aside from producing phospholipids, the ERalso produces cholesterol and ceramide.

Ceramide made by condensing the amino

acid serine with a fatty acid to form the amino

alcohol sphigosine; a second fatty acid is the

added to form ceramide.

transported to the GA, where it serves as

the precursor for the synthesis of two types

of lipids: oligosaccharide chains are added to

form


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glycosphingolipids, and phosphocholine head

groups are transferred from phosphatidylcholine

to other ceramide molecules to form

sphingomyelin.

Both glycosphingolipids and sphingomyelin are produced

relatively late in the process of membrane synthesis. Because both are produced by enzymes exposed to the

GA, they are exclusively found in the noncytosolic half of the

lipid bilayers that contain them in contrast to the

phospholipid synthesis which occurs in the cytosolic half of

the lipid bilayer.


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Recall that the plasma membrane and themembranes of the Golgi apparatus,lysosomes, and endosomes all form part of a

membrane system that communicates withthe ER by means of transport vesicles thattransfer both proteins and lipids.

Mitochondria, plastids and peroxisomes arenot included since they require differentmechanisms.


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PHOSPHOLIPID EXCHANGE PROTEINS

Also called phospholipid transfer proteins.

Shown in vitro to have the ability to transferindividual phospholipid molecules between

membranes. functions by "extracting" a molecule of the

appropriate phospholipid from a membrane

and diffusing away with the lipid buried withinits binding site.

Recognizes only specific types of proteins.


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PROTEIN EXCHANGE PROTEINS...

Act to distribute phospholipids at randomamong all membranes present.

In principle, a random exchange process can

result in a net transport of lipids from a lipid-rich to a lipid-poor membrane, allowingphosphatidylcholine and phosphatidylserinemolecules, for example to be transferredfrom the ER where they are synthesized, to amitochondrial or peroxisomal membrane.


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THE END!

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