Intracellular compartments and protein sorting

Question 1

What are some of the functions of Organelles ?

Answer 1

Ribosomes - Synthesis Proteins

Golgi apparatus- Modify proteins adding lipids

Mitochondria and chloroplast - generate ATP

Lysosomes : Degrade intracellaur organelle and particles up taken from outside the cells

Endosomes: Uptake material from the outside of the cell

Peroxisomes - Are the compartment implicated in oxidative reactions

Question 2

How do the proteins leave the ribosomes and reach the organelles where they have to reside?
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Answer 2

Protein synthesis occur in ribosomes Proteins leave the ribosomes .

Proteins are “translated” by ribosomes according to genetic information carried by mRNA

Ribosomes are organelles that are not membrane-enclosed. They synthetize both soluble and integral membrane proteins.

Question 3

What are the key challenges for Protein Transport into specific organelle ?

Answer 3

Topological challenges and Specificity challenge

Question 4

What is Topological challenge ?

Answer 4

Ribosomes only on cytosolic face  nascent (newly synthesised) proteins must cross the membrane, at least in part. 

Membrane-bound proteins must, in addition, be embedded in the plasma membrane, with the correct orientation of each TM helix, luminal and cytoplasmic domains 

Membrane integrity must not be disrupted in the process

Inside lumen of endomembrane system topically equivalent to the extra cellular space

Equivalent compartments In secretory and endocytic pathways in a eukaryotic cell

Equivalent if they communicate with another in the sense in which molecules can get one to another without having to cross a membrane

Question 5

Which organelles do not take part in communication ?

Answer 5

Mitochondria and plastids

Question 6

What about the specificity challenge ?

Answer 6

Proteins much reach correct destination without scrambling the content of different compartments

Each protein much reach the right organelle

With each organelle having a specific function with characteristic proteins and enzymes carry out specific reactions .

Question 7

What are the two sites of Protein synthesis ?

Answer 7

Cytosol and Rough endoplasmic reticulum being the only two sites of protein synthesis

The rough ER has many ribosomes bound its cystolic space .

Proteins synthetized by cytosolic ribosomes stay in the cytosol.

Proteins destined to the ER are transported into the ER as they are synthetized on the ribosomes.

Proteins destined to other organelles are are synthetized in the ribosomes of the rough ER and then transported to the right organelle.

Question 8

What are the three different ways in which proteins move from one compartment to another ?

Answer 8

Gated Transport

Protein Translocation

Vesicular Transport

Question 9

Gated Transports ?

Answer 9

only applicable to transport of proteins through the nucleus. Proteins reach the nucleus by passing through nuclear pore complexes on the nuclear membrane.

The nuclear pore functions as a selective gates to actively transport specific macromolecules and macromolecular assembling between topically equivalent space , allows free diffusion of smaller molecules

Proteins reach nucleus by passing through nuclear pore

Question 10

Protein Translocation ?

Answer 10

protein translocators transport the proteins across the membrane from the cytosol to the specific organelle. (Used by integral membrane proteins).

Transmembrane protein translocations directly trans port specific proteins across a membrane from the cytosol into a space which is topologically distinct

Transported protein molecules must unfold to shake through trans locators

Initial transport of selected proteins from the cytosol into the ER lumen or mitochondria for example occur in the way

Question 11

Vesicular Transport ?

Answer 11

membrane-enclosed transport intermediates ferry proteins from one topologically equivalent compartment to another. (Used by soluble proteins).

ER-Golgi apparatus : Because the transported proteins do not cross a membrane , this transport can move proteins only between compartments topologically equivalent

Transport vesicles , become loaded with cargo of molecules derived from lumen compartment

Bud and pinch off membrane discharge their cargo into 2 compartments by fusing with membrane enclosing that compartment

Question 12

What is Signal Sequence ?

Answer 12

Is a short amino acidic sequence which specify in which organelle certain protein is destined .

Specify intracellular transport of proteins
]Such as Return to ER

• Lys-Asp-Glu-Leu-COO

Question 13

Process of Cytosol to Endoplasmic reticulum ?

Answer 13

ER has a central role