MBS604 P1L2 ER, Golgi, and Endosomes

Eukaryotic cells contain a basic set of _______ _______

Membrane-enclose organelles

A cell from the lining of the intestine contains the basic set of membrane-enclosed organelles found in most animal cells. The ____, _____ _____, ____ ____, _____, _____, _____, and _______ are distinct compartments separated from the cytosol by at least one selectively permeable membrane.

Nucleus, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, endosomes, mitochondria, and peroxisomes

Endoplasmic reticulum (ER): main function

Synthesis of most lipids

Synthesis of proteins for distribution to many organelles and to the plasma membrane

Golgi apparatus: main function

Modification, sorting, and packaging of proteins and lipids for either secretion or delivery to another organelle

Endosomes: main function

Sorting of endocytose material

Endoplasmic reticulum: percentage of total cell volume and approximate number per cell

12%

1

Golgi apparatus: percentage of total cell volume and approximate number per cell

3%

1

Endosomes: percentage of total cell volume and approximate number per cell

1%

200

What is the most extensive membrane network in eukaryotic cells?

Endoplasmic reticulum (ER)

All ER compartments are interconnected by a _____ _____ ____ so that the organelle has only one large, high convoluted and completely arranged lumen (internal space)

Single continuous membrane

The ER membrane is contiguous with the ___ ____ ____

Outer nuclear membrane

ER with ribosomes bound to its cytosolic surface is called _____ _____

The rough ER (RER)

*Additional ribosomes can be found free in the cytosol

The ER that lacks ribosomes is called ____ ____

Smooth ER (SER)

The lumen of the ER is connected to the ____ ____ ____

Outer nuclear membrane

What are the functions of rough ER?

Synthesize, process, modify, and transport proteins

Proteins that are fully translated in the cytosol by ___ ___ end up in the ____ but not in the ____ ____

Free ribosomes

Cytosol, mitochondria, nucleus, etc

Transport vesicles (derived from ER and Golgi)

Many cytosolic ribosomes bind to each mRNA molecule, forming an efficient _____, to translate proteins without a ____ ____

Polyribosome

Targeting signal (such as ribosomal proteins)

Membrane-bound ribosomes

Translate proteins that contain an ER targeting signal sequence (hydrophobic amino acids)

*Directs protein translation to the ER and form a polyribosome on each mRNA

Most proteins destined to enter the ER begin to cross the membrane while ____ ____ ____

Still being synthesized

____ ____ made on the RER are released into the ER lumen.

Soluble proteins

The ER signal peptide will be cleaved from the growing protein by a ___ ___ to ___ ___ ___

Signal peptidase

Release mature protein

Transmembrane protein

In addition to the ER signal sequence, transmembrane proteins also contain a stop-transfer hydrophobic sequence

Stop-transfer hydrophobic sequence

When this sequence enters the protein translocation, the growing polypeptide chain is discharged into the lipid bilayer. The ER signal sequence is cleaved off, leaving the transmembrane protein anchored in the membrane.

N-linked glycosylation

Many proteins are glycosylated on asparagines (mostly in asparagine-X-serine/threonine sequence) in the ER lumen (proteins modified here). Dolichol-linked oligosaccharide (14-sugar) chain is transferred to the nitrogen atom of an asparagine side chain by oligosaccharyl protein transferase.

O-linked glycosylation

On serines or threonines happens in the Golgi

Accumulation of misfolded proteins in the ___ ___ triggers an ___ ___ ___

ER lumen

Unfolded protein response (UPR)

The misfolded proteins are recognized by several types of transmembrane sensor proteins in the ____ ____ , each of which activates a different component of the ___

ER membrane

Unfolded protein response (UPR)

Some sensors stimulate the production of transcription regulators that ___ …

Activate genes encoding chaperones or other proteins involved in ER quality control.

Another sensor can also ___ ____ ___, reducing the flow of proteins through the ER

Inhibit protein synthesis

Transport vesicles bud from ER and fuse with ___ ___, carrying and transporting soluble proteins and membrane components from ___ to ____

Golgi apparatus

ER to Golgi

Transport vesicles bud from Golgi apparatus, carrying and transporting soluble proteins and membrane components from ___ to ___ ___ or to ___ ___

From Golgi to plasma membrane or to early endosomes

Early endosomes by endocytosis fuse with transport vesicles and eventually develop into ____

Lysosomes

Endosome biogenesis

Clathrin-coated (+adaptor 2) vesicles formed by endocytosis transport selected cargo molecules from the plasma membrane to endosomes for disposal

*Clathrin = protein needed to deform plasma membrane

Type of coated vesicle: clathrin-coated

Origin: Golgi apparatus

What is the destination?

Golgi apparatus —> lysosome (via endosomes)

Type of coated vesicle: clathrin-coated

Origin: Plasma membrane

What is the destination?

Plasma membrane —> endosomes

Type of coated vesicle: COPII-coated

Origin: ER

What is the destination?

ER —> Golgi cisterna

Type of coated vesicle: COPI-coated

Origin: Golgi cisterna

What is the destination?

Golgi cisterna —> ER

Exocytosis

Transport vesicle docking depends on Rab (RAS-associated binding) proteins, tethering proteins, and SNAREs (SNAP RECEPTORs)

Functions of smooth ER (scarce in most cells) are to ___ ___, ___, and ___, etc.

Synthesize lipids, phospholipids, and steroids, etc.

Smooth ER (SER) lacks ribosomes and functions in ___ …

Lipid synthesis (but not catabolism), production of steroid hormones, and detoxification

New membrane phospholipids are synthesized by ___ …

Enzymes bound to the cytosolic side of the ER membrane

SER also regulates cytosolic ___ ion concentration. The major purpose of the Ca2+ pumps (on the SER and plasma membrane) is to keep cytosolic concentration of Ca2+ ___.

Calcium

Low

When Ca2+ does move into the cytosol, it alters the activity of many proteins because ___…

Ca2+ is a powerful signaling molecule

___ is especially abundant in mammalian liver and gonad cells

SER

The Golgi apparatus is a ____ ____ consisting of an oriented stack of ___ ___ surrounded by a swarm of small vesicles.

Polarized structure

disc-shaped cisternae

___ is a major collection and dispatch station of proteins and lipids received from ER (by transport vesicles)

Golgi apparatus

The Golgi apparatus is polarized. It has two distinct faces:

A cis forming face and a trans or maturing face

The ___ ___ is closely associated with a transitional ER

Cis face

In secretory cells, the ___ ___ is the closest to the plasma membrane

Trans face

Cargo proteins from ER are further modified by ___ ___ and destined for secretion via ___ ___ ___ (e.g. membrane maintenance) and ___ ___ (e.g. responding to extracellular signals)

O-linked glycosylation

Unregulated constitutive exocytosis

Regulated exocytosis

Tagging a protein with ___ allows the resulting fusion protein to be tracked throughout the cell.

GFP
*The fusion protein labels the ER —> GFP fusion protein rapidly accumulates at ER exit sites —> the fusion protein then moves to the Golgi apparatus —> the fusion protein is delivered to the plasma membrane

Where does most new membrane synthesis take place in a eukaryotic cell?

In the endoplasmic reticulum

New membrane phospholipids are synthesized by enzymes bound to the ___ side of the ___ membrane

Cytosolic

Endoplasmic reticulum

You have generated antibodies that recognize the extracellular domain of the Ca2+ pump. Adding these antibodies to animal cells blocks the active transport of Ca2+ from the cytosol into the extracellular environment. What do you expect to observe with respect to intracellular Ca2+?

Ca2+ pumps in the endoplasmic reticulum membrane keep cytosolic calcium levels low

Ca2+ pumps in the plasma membrane and for the endoplasmic reticulum are important for:

Preventing Ca2+ from altering the activity of molecules in the cytosol

Which of the following statements about the endoplasmic reticulum (ER) is TRUE?

The ER is the major site for new membrane synthesis in the cell

Proteins to be delivered to the ER lumen are synthesized on the rough ER

Steroid hormones are synthesized on the smooth ER

The ER membrane is contiguous with the outer nuclear membrane

Proteins that are fully translated in the cytosol do not end up in:

ER, Golgi, and transport vesicles

Proteins that are fully translated in the cytosol and lack a sorting signal will end up in the:

Cytosol

In which cellular location would you expect to find ribosomes translating mRNAs that encode ribosomal proteins?

In the cytosol

After isolating the rough endoplasmic reticulum from the rest of the cytoplasm, you purify the RNAs attached to it. Which of the following proteins do you expect the RNA from the rough endoplasmic reticulum to encode?

Soluble secreted proteins

ER membrane proteins

Plasma membrane proteins

Which of the following choices reflects the appropriate order of locations through which a protein destined for the plasma membrane travels?

ER —> Golgi —> plasma membrane

Different glycoproteins can have a diverse array of oligosaccharides. Which of the statements below about this diversity is TRUE?

Oligosaccharide diversity comes from modifications that occur in the ER and the Goligi of the 14 sugar oligosaccharide added to the protein in the ER

Vesicles from the ER enter the Golgi at the:

Cis Golgi network

Cellls that are specialized for the secretion of proteins are likely to have which of the following features?

Enlarged endoplasmic reticulum

What is the function of endosomes?

Sorting and transport of endocytosed material

Coated vesicles transport material from ER to Golgi, or from Golgi to ER, or from Golgi to lysosomes, or from plasma membrane to endosomes. True or false?

True

Which of the following organelles is not part of the endomembrane system?

Mitochondria

Chloroplasts