Endomem Sys & Protein Sorting (BIO201 Review)

Question 1

What is the endomembrane system?

Answer 1

The endomembrane system is the set of membranes that form a single functional and developmental unit, either being connected directly or exchanging material through vesicle transport.

I.e. a sys of interconnected organelles.

I.e. The organelles are connected by membrane bridges or communicate by budding off and fusing vesicles that transport substances b/w them.

Recall: endomem sys includes NE, ER, Golgi, Lysosomes/Vacuoles, Pmem, and Vesicles.

Question 2

What are the organelles of the endomem sys

Answer 2

Nuclear envelope, ER, Golgi, lysosomes/vacuoles, pmem, and vesicles that move b/w them.

Mnemonic: NEGLPV…?

Recall: The endomem sys is a set of membranes, thus includes nuclear envelope, not the entire nucleus.

Recall: endomem sys doesn’t include ribosomes, mitochondria, or chloroplasts.

Question 3

What is the diff b/w cytoplasm and cytosol?

Answer 3

Cytosol is jelly-like internal environment, surrounds organelles inside pmem.

Cytoplasm houses entire contents of cell, excluding the nucleus.

Question 4

T/F: The nucleus is not considered part of the cytoplasm.

Answer 4

True

The nucleus is not considered part of the cytoplasm

Question 5

What connects the endomem sys in plants?

Answer 5

Plasmodesmata. Makes endomem sys continuous b/w cells

Recall: Plasmodesmata and gap junctions (in animal cells) are both types of communicating cell junctions.

Question 6

What are exocytosis and endocytosis?

Answer 6

Exocytosis - Vesicle buds off fr endomem sys → fuses w pmem → empties contents into EC space or delivers protein to pmem.

Endocytosis - Material fr EC space brought into cell via vesicle; vesicle buds off pmem, fuses w internal organelle, delivers contents to organelle.

Question 7

Describe the structure and function of the nuclear envelope.

Answer 7

Nuclear Envelope

Structure - Dual mem (2 sep bilayers; perinuclear space in b/w). Both membranes are perforated/conn by nuclear pores, thru wh small mcules/ions can passively diffuse (large proteins and RNA req active xprt).

Function - Defines boundary of nucleus. Regulates movement of mcules in/out nucleus

Question 8

Describe the location, structure, and function of the ER.

Answer 8

Endoplasmic Reticulum

Location - Physically continuous w outer mem of nuclear envelope.

Structure - Interconn, v convoluted tubules and flattened sacs (cisternae); cisternal space (inside) called “lumen”.

Recall: convolutions help large organelle fit into cell, increases surface area.

Function - Produces (& transports) many lipids/proteins, i.e. site of synth of most mem lipids, synth of all xmem pro and pros destined for Golgi, lysos, or export.

Recall: The lumen is the continuous interior space (“cisternal space”) in euks. More specifically, in plants, it’s a fluid-filled compartment enclosed by thylakoid mem.

Rough ER (RER) - Studded w ribos wh synth pros, incl xmem, those for int organelles, and those for export.

Smooth ER (SER) - Site of lipid synth (FAs, plipids, steroid hormones). Also synths some carbs (monosacchs or smaller polysacchs), detoxifies meds/toxins, and stores calcium ions.

Note: Most carbs synthesized in Golgi.

Note: plipids may be synthesized on Rough ER. Check on this.

Question 9

Regarding plant cells, what is the lumen?

Answer 9

The lumen is the continuous interior space in euks. More specifically, in plants, it’s a fluid-filled compartment enclosed by thylakoid mem.

Question 10

How does the highly convoluted struc of ER contrib to its function?

Answer 10

More convolutions → incr surface area → more locations for lip/pro synth.

Also, allows large mem to fit in cell.

Question 11

Differentiate b/w the function of RER and SER.

Answer 11

RER is responsible for protein synth, while SER is responsible for FA/p-lipid synth.

Recall: Proteins synthesized on RER are destined for everywhere except mito/chloro, nucleus, or those that will remain in cytosol. I.e. proteins destined for ER mem/lumen, Golgi mem/lumen, lysosomes, pmem, or cell exterior (secretion)

Question 12

The _______ is a very convoluted series of interconnected tubules and flattened sacs.

Answer 12

The ER is a very convoluted series of interconnected tubules and flattened sacs (“cisternae”).

Recall: Responsible for most FA/p-lipid synth as well as synth of proteins destined for ER mem/lumen, Golgi mem/lumen, lysosomes, pmem, or cell exterior (secretion), i.e. protein destinations of co-translational transport.

Question 13

Where are transmembrane (integral) and peripheral proteins synthesized?

Answer 13

Transmembrane proteins are synthesized by ribosomes on RER.

Synthesis of peripheral proteins depends on target destination:

Peripheral proteins destined for cytoplasmic side of pmem are synthesized entirely on free/cytoplasmic ribosomes.

Peripheral proteins destined for EC side of pmem are synthesized on RER, transported to Golgi, and secreted.

Question 14

Describe the location, structure, and function of the Golgi Apparatus.

Answer 14

Location - Very close to ER, but not physically attached. Next stop for vesicles that bud fr ER.

Structure - Stacks of flattened mem sacs (cisternae) surrounded by vesicles (containing enzymes).

Functions:

Modify - Further modifies pro/lip produced by ER, e.g. glycosylation (sugars covalently linked to lipids or AAs as mcules move thru Golgi).

Sort - Sort pro/lip as they move to final destination; act like a routing station.

Carb Synth - Synthesize most of the cell’s carbs.

Question 15

T/F: Initial modification of proteins/lipids occurs on ER prior to transport to Golgi.

Answer 15

True

Golgi is responsible for further/final modification of proteins/lipids synthesized on ER.

Recall: Besides modification, Golgi also responsible for sorting pro/lip and carb synth.

Question 16

Where are the enzymes responsible for protein/lipid modification located within the Golgi?

Answer 16

The enzymes responsible for protein/lipid modification are located within the vesicles that surround cisternae.

Recall: Golgi is composed of stacks of flattened membranous sacs (cisternae) surrounded by vesicles.

Question 17

T/F: Proteins may also travel “in reverse” from Golgi to ER.

Answer 17

True

Proteins may also travel “in reverse” from Golgi to ER.

Reverse travel is important for retrieving pro in Golgi/ER that were accidentally moved forward; helps recycle pro.

Question 18

What are the three key funcs of Golgi?

Answer 18

Modify, sort, carb synth. Further modifies pro/lip received fr ER, e.g. glycosylation. Sorts modified pro/lip and directs to final destination. Synths most of cell’s carbs.

Question 19

Describe the location, structure, and function of lysosomes.

Answer 19

Location - In cytosol. A vesicle derived fr Golgi, and contain enzymes delivered fr Golgi.

Structure - A vesicle, wh is a small, mem-enclosed sac, containing many enzymes.

Function - Digestion, i.e. acts as garbage disposal; digestion of macromolecules

Proton pumps help maintain acidic pH (5) while broken-down mcules transported out of lysosome by transporters.

Question 20

T/F: Lysosomes are present in plant and animal cells.

Answer 20

True

Lysosomes are present in plant and animal cells

Question 21

Where do lysosomes originate?

Answer 21

The vesicle itself and the enzymes within are manufactured and budded into the cytoplasm by the Golgi.

H/e, the Golgi is responsible for further modification of proteins/lipids that originate on the ER.

Thus, an argument could be made that both the lipids that compose the lysosome’s membrane as well as the enzymes (proteins) contained within were both synthesized on the ER.

Question 22

What is protein sorting?

Answer 22

Protein sorting is process of sending pro to appropriate location. Process differs dep on whr pro produced.

Recall: All proteins begin translation on free/cytoplasmic ribosomes.

Question 23

Describe protein sorting for proteins produced on free ribosomes.

Answer 23

Proteins fully produced on free ribosomes are sorted after translation, i.e. “post-translational transport”.

• Typ contain signal peptides/seqs - hphobic AAs near N-terminus that enable recog/sorting.
  
  • Proteins w/o signal seq remain in cytosol.
  • Proteins containing signal seq at N-terminal are directed to mito/chloro.
  • Proteins winternal signal seq (nuclear localization signals), are directed to nucleus.

Summary: No signal seq → cytosol. N-terminal signal seq → mito/chloro. Internal signal seq → nucleus.

Recall: All proteins begin translation on free/cytoplasmic ribosomes. If polypeptide contains amino/N-terminal signal sequence that can be bound by a signal recognition particle (SRP), then translation halts and polypep is transported to ER, where synth resumes/completed.

Question 24

For proteins fully produced on free ribosomes, if there is no signal sequence present (w/i the AA sequence), where will the protein end up?

Answer 24

Proteins fully produced on free ribosome and w/o signal sequence remain in cytosol.

Recall: Part of post-translational transport

Question 25

For proteins fully produced on free ribosomes, if there is a signal sequence present at the polypeptide’s amino/N-terminal, where will the protein end up?

Answer 25

Proteins fully produced on free ribosome and with a signal sequence present at the amino/N-terminal are destined for mitochondria or chloroplasts.

Think: Amino as in BCAAs, which you take to support exercise, and both mito/chloro are assoc w energy.

Recall: All proteins begin translation on free/cytoplasmic ribosomes. If polypeptide contains amino/N-terminal signal sequence that can be bound by a signal recognition particle (SRP), then translation halts and polypep is transported to RER, where synth resumes/completed.

Here, amino/N-terminal signal sequence is not bound by SRP, so translation is completed on free ribosome, and protein is directed to mito/chloro.

Recall: Part of post-translational transport

Question 26

For proteins fully produced on free ribosomes, if there is an internal signal sequence w/i the polypeptide’s AA seq, where will the protein end up?

Answer 26

Proteins fully produced on free ribosome and containing internal signal sequences are destined for the nucleus.

Recall: internal signal sequences are also called “nuclear localization signals”.

Recall: All proteins begin translation on free/cytoplasmic ribosomes. If polypeptide contains amino/N-terminal signal sequence that can be bound by a signal recognition particle (SRP), then translation halts and polypep is transported to RER, where synth resumes/completed.

Recall: Part of post-translational transport

Question 27

What is the purpose of signal peptides/sequences?

Answer 27

Signal peptide seqs hphobic AA seqs, typ at N-terminus, wh allow protein produced on free ribos to be recog/sorted.

• Recall: All proteins begin translation on free/cytoplasmic ribosomes.
• If signal seq at N-terminal can be bound by SRP, then xl halts and polyp is transported to RER, where synth resumes/completed (co-translational transport).
• If signal seq at N-terminal cannot be bound by SRP, then xl is completed on free ribosome, and protein is directed to mito/chloro (post-translational transport).
  
  • No signal seq → cytosol. Amino/N-terminal seq → mito/chloro. Internal signal seq → nucleus.

Question 28

Summarize protein sorting for proteins produced on ER

Answer 28

Proteins produced on ER are sorted during translation (co-translational transport).

• These proteins are destined for ER/Golgi mem/lumen, or any shipping destination fr Golgi (lysosome, pmem, or cell exterior); also incl xmem proteins and peripheral mem proteins assoc w EC side of pmem.
• Begin translation on free ribosome (like all proteins), then signal seq (N-terminal) directs ribo to ER → thru mem channel → into ER lumen.
• If polyp contains no other signal seq: continues to be fed into ER lumen as it is synth’d by now-mem-bound ribo.
• If polyp contains another signal seq (“stop-transfer seq” or “mem anchor seq”): becomes part of ER mem (xmem) as it is synth’d by now-mem-bound ribo, or transported to pmem after xl.

Question 29

T/F: all proteins begin translation on free/cytoplasmic ribosome.

Answer 29

True

Even proteins synthesized on ER begin translation on a free ribosome. If destined to complete translation on ER, SRP will bind signal seq on amino/N-terminal during translation (co-translational transport).

Question 30

What are signal-recognition particles (SRPs), and how do they relate to protein sorting?

Answer 30

Signal-recognition particles (SRPs) bind to signal seq on amino/N-terminal of growing polypeptide (on free ribosome); halts translation.

Recall: all proteins produced on ER (whether destined for ER mem/lumen, golgi mem/lumen, or lysosome/pmem/export) begin translation on free ribo and then signal seq (on amino/N-terminal) directs ribo to ER.

Mechanism:

Polypep begins translation on free ribosome

SRP binds to signal seq on amino/N-terminal of growing polypep → halts translation

SRP (w attached ribosome and partial polypep) binds to SRP receptor on ER membrane

SRP Receptor brings SRP (w attached ribosome and partial polypep) to associated xmem channel protein in ER mem

SRP dissociates, and polypep synth resumes, w growing polypep chain threaded thru xmem channel.

Events from this point forward depend on whether signal seq on amino/N-terminal of polypep can be cleaved by signal peptidase as well as presence of additional, internal signal seq in polypep.

Question 31

Describe the mechanism of signal-recognition particles (SRP).

Answer 31

Mechanism:

Polypep begins translation on free ribosome

SRP binds to signal seq on amino/N-terminal of growingn polypep → halts translation

SRP (w attached ribosome and partial polypep) binds to SRP receptor on ER membrane

SRP Receptor brings SRP (w attached ribosome and partial polypep) to associated xmem channel protein in ER mem

SRP dissociates, and polypep synth resumes, w growing polypep chain threaded thru xmem channel.

Events from this point forward depend on whether signal seq on amino/N-terminal of polypep can be cleaved by signal peptidase as well as presence of additional, internal signal seq in polypep.

If amino/N-terminal seq can be cleaved and no other internal signal seq present, then translation continues w polypep threaded into ER lumen, and protein is destined to remain in ER lumen, to remain in Golgi mem/lumen, or any shipping destination from Golgi (lysosome, pmem, or cell exterior).

If amino/N-terminal seq cannot be cleaved (thus, “signal anchor seq”) and additional, internal signal seq present (stop-transfer or mem anchor seq), then translation continues until anchor seq encountered → xmem channel releases protein into ER mem.

Question 32

For proteins that begin translation on a free ribosome but are directed to ER (via signal seq on amino/N-terminal), additional, internal signal sequences are called _________.

Answer 32

For proteins that begin translation on a free ribosome but are directed to RER (via signal seq on amino terminal), additional signal sequences are called stop-transfer or membrane anchor sequences.

Recall: signal-anchor seqs are the name of the amino/N-terminal seq when it cannot be cleaved off by signal peptidase (after free ribo binds to ER xmem protein channel and polypep fed into ER lumen and ultimatley ER mem).

Question 33

For proteins that begin translation on a free ribosome and are directed to RER (via signal seq on amino/N-terminal), the presence of an additional seq - a stop-transfer or membrane anchor seq - indicates what target destination?

Answer 33

For proteins that begin translation on a free ribosome and are directed to RER (via signal seq on amino terminal), the presence of an additional seq - stop-transfer or membrane anchor seq - indicates a protein destined to become part of ER membrane.

Recall: signal-anchor seqs are the name of the amino/N-terminal seq when it cannot be cleaved off by signal peptidase (after free ribo binds to ER xmem channel and polypep fed into ER lumen).

Question 34

What is transmembrane protein targeting?

Answer 34

Transmembrane protein targeting refers to polypeptides which begin translation on a free ribosome and are directed to xmem channel protein on ER (via signal seq on amino/N-terminal → SRP → SRP receptor), and contain an additional signal seq - a stop-transfer or membrane anchor seq - that directs protein to be embedded in ER.

Question 35

T/F: Mitochondria and cholorplasts are not part of endomembrane system.

Answer 35

True

Question 36

T/F: Translation of all proteins begins in the cytosol.

Answer 36

True

Only minor exceptions, such as a few proteins synthesized in mito/chloro

Question 37

What are the possible protein destinations of post-translational transport, i.e. protein is fully produced prior to transport?

Answer 37

The possible protein destinations of post-translational transport are: Mitochondria, chloroplasts, peroxisomes, and the nucleus.

Note: all are non-endomembrane organelles

Recall: Protein fully produced on free ribosome and…

• …No signal seq → remains in cytosol, e.g. periphery membrane proteins assoc w cytoplasmic side of pmem
• …amino/N-terminal signal seq → mito/chloro
• …internal signal seq (nuclear localization signal) → nucleus
• …end/C-terminal signal seq (peroxisomal targeting signal) → peroxisome

Question 38

What are the possible protein destinations of co-translational transport, i.e. protein is fully produced prior to transport?

Answer 38

The possible protein destinations of co-translational transport are: ER mem/lumen, Golgi mem/lumen, and all shipping destinations from Golgi (lysosomes, pmem, and cell exterior)

Recall: includes all xmem proteins as well as peripheral mem proteins assoc w EC side of pmem

Note: all are endomembrane organelles; cell exterior not technically part of endomem sys, but vesicles are.

Question 39

Is the amino/N-terminus of a polypeptide chain located at the beginning or end, i.e. translated first or last?

Answer 39

The amino/N-terminus of a polypeptide chain is located at the beginning, meaning it’s translated first.

Signal peptides/sequences at the amino/N-terminus of a growing polypeptide may be recognized by signal recognition particles (SRPs) translation and protein sorting.

Signal seqs are typ located at amino/N-terminus

Question 40

T/F: Signal peptides/sequences are comprised of a series of hydrophobic amino acids.

Answer 40

True

Question 41

Proteins destined for _______ organelles are typ synthesized in the cytosol (on free ribosome) while proteins destined for ________ organelles are typ synthesized on ER.

Answer 41

Proteins destined for non-endomembrane organelles are typ synthesized in the cytosol (on free ribosome) while proteins destined for endomembrane organelles are typ synthesized on ER.

Recall: All proteins begin translation on free/cytoplasmic ribosome.

Recall: Proteins bound for mito, chloro, peroxisomes, and nucleus are typ made in cytosol and sorted after translation is complete (post-translational transport).

Recall: Proteins bound for ER mem/lumen, Golgi mem/lumen, and shipping destinations of Golgi (lysosomes, pmem, and cell exterior) are sorted during translation (co-translational transport).

Note: Cell exterior isn’t part of endomembrane system, but vesicle are (shipped from Golgi).

Question 42

Is the carboxyl/C-terminus of a polypeptide chain located at the beginning or end, i.e. translated first or last?

Answer 42

The carboxyl/C-terminus of a polypeptide chain is located at the end, meaning it’s translated last.

Signal peptides/sequences at the carboxyl/C-terminus of a polypeptide (e.g. peroxisomal targeting signal) are used to direct protein sorting.

Signal seqs are typ located at amino/N-terminus.