BIO130H1 Lecture Notes - Lecture 18: Translocon, Secretion, Cell Membrane

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6 Apr 2019

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Document Summary

A stretch of the amino acid sequence of a protein that directs the protein to the correct location in the cell. Each signal sequence specifies a specific destination in the cell. Eg. to the nucleus, mitochondria, er, peroxisomes, etc. Signal sequences are recognized by sorting receptors that take proteins to the destination ( srps for er ) Really highlights the power of the genome dna sequence tells the proteins where to go. Signal peptidases can remove the signal sequences that are found on the n-terminus of proteins. Signal sequences can also be internal stretches of amino acids which remain part of the protein ( internal signal sequence or internal start transfer sequence ) Signal sequences on the amino-terminal (n) end directs the protein to the er. Protein is inserted through the membrane by a translocon protein. Signal sequence is cleaved, left behind in er; secreted protein in the er lumen. Secreted protein moves in transport vesicles via the secretory pathway.

Related Questions

1. The histones that form the core of the nucleosome get into the nucleus by which of the following?

a) Passive diffusion through nuclear pores

b) Selective and facilitated active transport through nuclear pores

c) Ribosomes located in the nucleus synthetize histones in the nucleus

d) Histones are made at the ER membrane and then diffuse to the nucleus

2. Membrane proteins that belong to the golgi, lysosome, or plasma membrane first pass through which organelle before reaching their final subcellular destination?

a) The golgi apparatus

b) The smooth ER

c) The nucleus

d) The ER membrane

e) The ER lumen

3. Where in the cell would a ribosomal protein be located if it has had its nuclear export signal inactivated (non-functional or non-existent).

a) The protein would be found in the ER lumen

b) The protein would remain in the nucleus

c) The protein would be secreted

d) The protein would remain in the cytoplasm

e) The protein would be in the ER membrane

4. Where would be the final destination of a protein if it contained a NLS anywhere in the protein sequence (and which is exposed to the cytoplasm after protein folding) but also contained an ER signal sequence at the carboxy terminus?

a) The ER lumen

b) The nucleus

c) The cytoplasm

d) The inner nuclear membrane

e) The ER membrane

5. The sequence K-D-E-L serves as a retention/retrieval signal for ER proteins.

a) True

b) False

6. The following are true for both cell membrane proteins and secretory proteins EXCEPT?

a) Precursors of the mature proteins contain signal sequence.

b) Both types of proteins contain stop-transfer sequences

c) Both types of proteins enter the ER while being synthetized

d) Both types of proteins pas through the golgi apparatus

e) Both are carried by clatherin-coated vesicles

7. If you isolate a regulated secreted protein from a secretory/storage vesicle and inject it into the cytosol, is will?

a) Be secreted through channels in the plasma membrane

b) Be taken up into secretory vesicle and secreted

c) Remain in the cytosol until it is degraded

d) Be taken up into the rough ER and follow the secretory pathway

goldmanatee250

Imagine that you have engineered a set of genes , each encoding a protein with a pair of conflicting signal sequences that specificy different compartments. If the genes were expressed in a cell, what signal would win and why?

1) A) Signals for import into ER would beat signals for import int the nucleus. What is the reasonable explanation?

1) The nuclear transport receptor is more efficient than the signal recognition particle (SRP)

2) The ER signal near the N-terminus will be exposed before the nuclear signal during protein synthesis and thus recognized by SRP before the nuclear signal is even available

3) the nuclear signal near the middle of the sequence will be exposed before the ER signal during protein synthesis and thus recognized by the nuclear transprot protein before the ER signal is active.

4) the SRP is more efficient than the nuclear transport receptor.

B) Signals for import into ER would beat signals for import into peroxisomes. What is the reasonable explanation?

1) The peroxisome transport proteins are more efficient than the signal recognition particle (SRP)

2) The ER signal near the N-terminus will be exposed before the peroxisome signal during protein synthesis and thus recognized by SRP before the peroxisome signal is even available

3) peroxisome proteins are transported by vesicles not by transport proteins.

4) the SRP is more efficient than the peroxisome transporter.

C) Signals into the mitochondria and retention into the ER? Which would win? Reasonable explanation as to why?

1)Mitochondria

2)ER

and

1) The mitochondrial transport protein is more efficient than the signal recognition particle (SRP)

2) The ER signal near the N-terminus will be exposed before the mitochondrial signal during protein synthesis and thus recognized by SRP before the mitochondrial signal is even available

3) the mitochondrial signal near the N-terminus of the sequence will be exposed before the ER retention during protein synthesis and thus recognized by the SRP before the ER signal is active

4) the signal for retention of a protein in the ER is at the C-terminus and not recognized until import into the ER begins