Biology 2382B Lecture Notes - Lecture 13: Secretion, Secretory Protein, N-Terminus

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5 May 2018

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Lecture 04: Protein Synthesis and Transport

Protein Sorting and Targeting

- Typical mammalian cell has 10,000 proteins that need to be localized correctly

- Newly made peptides must be directed to the correct destination

- Targeting:

- Direct proteins to the right destinations (organelles)

- During or after synthesis

- Sorting:

- Direct proteins to the secretory pathway (ER, Golgi, lysosomes)

*note: sorted proteins are first targeted to the ER and then further processed and sorted to golgi

apparatus or secreted to lysosomes.

General Principles of Protein Synthesis, Targeting and Sorting

- Proteins that remain in the cytosol or targeted to organelles (i.e. proteins that remain in the cell) are

synthesized by cystolic/free ribosomes. Proteins that are targeted to intracellular organelles (ER,

mitochondria, chloroplasts, peroxisomes, and nucleus) have a specific signal sequence (AA sequence on

amino terminus of proteins).

- Proteins that reside in the ER or are sorted to plasma membrane, golgi complex, and lysosomes are

synthesized by ribosomes attached to ER (rough ER).

protein ER, mitochondria, chloroplast, peroxisomes, nucleus

protein ER plasma membrane, golgi apparatus, lysosome

- Accordingly, there are 2 major protein-sorting pathways:

1. Non-Secretory Pathway  protein goes from ER to cytosol, or intracellular organelles

2. Secretory Pathway secreted outside of the cell

ER Structure

- ER is an extension of the nuclear membrane (rough ER closer to nucleus)

- ER = uninterrupted membranous tubules and vesicles separated from cytoplasm

- RER has ribosomes on the tubules (cisterna - cisterna are stacked)

- Inner parts of the tubes is the ER lumen

Q: How can we prove that secretory proteins must first be targeted to the ER? A: Experiments...

1) Add radio-labelled AA to cell which are incorporated into the synthesized secretory protein

2) Homogenization of cells

- break plasma membrane, partially disrupt ER

- ER reforms into small vesicles called microsomes (mini-ERs) w/attached ribosomes

3) Centrifugation

- differential then density to isolate microsomes

4) Split isolated microsome

targeted

sorted

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Document Summary

Typical mammalian cell has 10,000 proteins that need to be localized correctly. Newly made peptides must be directed to the correct destination. Direct proteins to the right destinations (organelles) Direct proteins to the secretory pathway (er, golgi, lysosomes) *note: sorted proteins are first targeted to the er and then further processed and sorted to golgi apparatus or secreted to lysosomes. General principles of protein synthesis, targeting and sorting. Proteins that remain in the cytosol or targeted to organelles (i. e. proteins that remain in the cell) are synthesized by cystolic/free ribosomes. Proteins that are targeted to intracellular organelles (er, mitochondria, chloroplasts, peroxisomes, and nucleus) have a specific signal sequence (aa sequence on amino terminus of proteins). Accordingly, there are 2 major protein-sorting pathways: non-secretory pathway protein goes from er to cytosol, or intracellular organelles, secretory pathway secreted outside of the cell. Er is an extension of the nuclear membrane (rough er closer to nucleus)

Related Questions

The rough ER is the site of synthesis of many different classes of membrane protein. Some of these proteins remain in the ER, whereas others are sorted to compartments such as the Golgi apparatus, lysosomes, and the plasma membrane. One measure of the difficulty of the sorting problem is the degree of âpurificationâ that must be achieved during transport from the ER. Are proteins bound for the plasma membrane common or rare among all ER membrane proteins? A few simple considerations allow one to answer this question. In a typical growing cell that is dividing once every 24 hours, the equivalent of one new plasma membrane must transit the ER every day. If the ER membrane is 20 times the area of a plasma membrane, what is the ratio of plasma membrane proteins to other membrane proteins in the ER? (Assume that all proteins on their way to the plasma membrane remain in the ER for 30 minutes on average before exiting, and that the ratio of proteins to lipids in the ER and plasma membranes is the same.) PLEASE DUMB THIS DOWN

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

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Which compartments are involved in vesicular transport? How are transport vesicles formed and targeted appropriately? What proteins are involved and what are their roles? What kinds of modifications are added to proteins in the ER? What determines whether a protein stays in the ER or moves on? Be sure to understand how vesicles fuse with their target membranes, and the consequences of fusion for the vesicleâs cargo as well as the proteins and lipids that are in the membrane. Know the structure of the Golgi apparatus and its role in intracellular trafficking. Understand the secretory process, endocytosis, exocytosis, function of the endosomes, and function of the lysosomes. Understand why endocytosis happens and how it can be regulated to take up specific molecules.

Biology 2382B Lecture Notes - Lecture 13: Secretion, Secretory Protein, N-Terminus

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