BIOL 445 Lecture 10 Terms [UNFINISHED]

What type of transport is used to get proteins into the ER?

transmembrane transport

How much does ER account for of the total membranes in an average animal cell?

~50%

How much does ER lumen account for of cellular volume?

~10%

Is the membrane of the ER continuous with the outer membrane of the nuclear envelope?

true

The Rough ER…

is studded with ribosomes on its cytosolic face

What is Rough ER involved with?

co-translational import of proteins

The Smooth ER…

is where vesicles bud

What is the part of the smooth ER that vesicles bud off to go to the Golgi called?

transitional ER

What roles does the smooth ER do?

detoxification, steroid production, calcium sequestration

What is the special smooth ER called in muscle cells?

sarcoplasmic reticulum

What are the two ways that proteins translocate into the ER?

post-translationally or co-translationally

What does Post-translational mean?

polypeptide is translocated after it is translated

What does Co-translational mean?

polypeptide is being translocated whilst it is being translated

How does post-translational translocation work?

chaperones bind to proteins to keep them unfolded and are pulled into the ER through translocator and ATPase

How does co-translational translocation work?

protein is translocated into ER as it is being translated by the ribosome

Does co-translational translocation require chaperones?

false

Does post-translational translocation require chaperones?

true

What is integral membrane proteins?

ultimately targeted to the plasma membrane or organelle’s membrane

What is soluble proteins?

destined to be in the lumen of an organelle or secreted out the cell

In bacteria, what does the post-translational translocation use?

SecA ATPase

In eukaryotes, what does the post-translational translocation use?

Sec complex

Does bacteria have ER?

false

What does BiP in eukaryotes do?

protein in ER lumen that pulls polypeptide into lumen

What kind of chaperone is BiP in eukaryotes?

Hsp70

Does BiP use ATP hydrolysis to power active transport into the ER lumen

true

How does co-translational translocation begin?

ER signal sequence recognized as it is translated 5’ to 3’

How is the protein released into the ER lumen in co-translational translocation?

protein elongates into lumen, and signal peptide is then cleaved off

Is the signal sequence pushed into the ER lumen in co-translational translocation?

false

Is the translocated closed unless there is a ribosome bound to it and it is actively translocating?

true

How is the ER signal sequence recognized in co-translational translocation?

signal-recognition particle (SRP)

How does SRP recognize the signal sequence?

hydrophobic residues in signal sequence bind to the hydrophobic pocket of the SRP

Does the SRP only recognize signal peptides that are still associated with the ribosome?

true

What happens when SRP binds to the ribosome?

blocks elongation factor binding, which pauses translation

What does binding of the SRP to the signal sequence expose?

a binding site for the SRP receptor

What does the SRP receptor do?

targest the complex to the translocator point

Why does the ER become studded with ribosomes?

because of the growing polypeptide chain that holds them there, but are not actually bound to ER

Where is free ribosome cycle located?

in the cytosol

Where is the membrane-bound ribosome cycle located?

on the surface of the ER

What is the free ribosome cycle?

proteins translated in the cytosol, and after they are done they return to the common pool

What is the membrane bound ribosome cycle?

goes through the SRP cycle, and is transferred back to common pool after translating

Why is the ribosome free to go to the common pool after SRP cycle?

it is bound to just the signal sequence, not the translocator itself

What is the translocator in this process called?

Sec61

What kind of channel is Sec61?

dynamically gated channel

How many subunits does Sec61 have?

3 subunits

Why is it important that Sec61 is gated?

blocks molecules from passing through channel when it is not actively translocating

What leads to the opening of the channel on Sec61?

when channel binds to signal peptide, it pushes plug to one side

How does co-translational translocation work for soluble proteins?

SRP binds and goes through cycle, then targets the complex to the translocase and translates while being translocated through Sec61

How does the co-translational translocation work for a single pass protein for the N-terminal signal sequence?

SRP recognizes protein and is targeted to translocase, where it keeps transferring until a hydrophobic region opens the seam on the side of the translocase, which stops the transfer

Does the co-translational translocation for a single pass protein leave the N-terminus in the ER lumen and the C-terminus in the cytosol?

true

What kind of signal sequence is present for proteins that need to be flipped upside down, so the C-term is on the lumenal face?

internal signal sequence

How does the internal signal sequence work in co-translational translocation?

polypeptide translated in cytosol and somewhere in the middle is the ER signal sequence, where is it is then recruited to the translocator

What sequence will end up in the cytosolic side in co-translational translocation?

the more positive sequence

What kind of signal do multi pass proteins use to start translocation?

internal signal sequence

Is the sequence more positive before the signal sequence than after, so the C-term will get transferred?

true

In co-translational translocation in multi pass proteins, if the C-term is in the cytosol,

the signal right before it is a STOP

In co-translational translocation in multi pass proteins, if it C-term is in the ER,

the signal right before it is a START

What causes loops in co-translational translocation of multipass proteins?

pairs of start/stop sequences

Where is the signal located in ER-tail anchored proteins?

the C-term

Can the SRP be used in ER-tail anchored proteins?

no

What protein does ER-tail anchored proteins use to get it targeted to the Get1-Get2 receptor?

Get3 ATPase

What does Get1-Get2 receptor do?

insert the signal sequence into the membrane, proteins are not passed the membrane

What is protein glycosylation?

covalent addition of sugar groups

What is a protein called after it is glycosylated?

glycoprotein

What is the enzyme responsible for connecting the protein and sugar called?

oligosaccharide transferase

Why is glycosylation of ER proteins so common?

the oligosaccharide is used for protein folding

What does the chaperones in the ER lumen do in regards to oligosaccharide?

bind to oligosaccharide to help unfold misfolded proteins

What is calnexin?

membrane-bound chaperones in the ER lumen

What is calreticulin?

soluble chaperones in the ER

Is the protein glycosylated as it is translocating?

yes

What happens when the oligosaccharide is trimmed to 1 terminal glucose?

it is recognized by the chaperone, and once bound, a glucosidase cleaves off the last glucose and releases protein from chaperone

If a protein is not folded properly in the ER…

glucosyl transferase puts terminal glucose back on and goes through cycle again with chaperone

What percentage of proteins never fold correctly?

~80%

How does cell know when to stop trying to fold a protein (if it never folds correctly)?

ER-associated degradation (ERAD)

If a protein has not folded correctly by the time that the mannose sugar is removed…

lectin recognizes removal, and targets protein to translocator for exporting protein, where the ubiquitinated polypeptide is targeted to and degraded by proteasome

What is the Unfolded Protein Response?

accumulation of unfolded proteins in the ER triggers stress response, where it increases number of chaperones in ER and gets rid of unfolded proteins via ERAD

What is IRE1 in Unfolded Protein Response?

sensing of unfolded proteins leads to multimerization of this protein kinase, which leads to autophosphorylation, which leads to increased transcription of genes encoding ER chaperones and retrotranslocation machinery

What is PERK in Unfolded Protein Response?

unfolded proteins lead to oligomerize and autophosphorylation, which leads to reduction of protein synthesis and selective translation of a different transcriptional regulator

What is ATF6 in Unfolded Protein Response?

unfolded protein is sensed and sent to the Golgi where it is proteolyzed

Is the ER membrane the major site of lipid synthesis?

true

How do phospholipids get to lumenal face?

through scramblases, which scrambles the phospholipids to both sides of the ER membrane

Is ER also the site of cholesterol and ceramide synthesis?

true