Intracellular Compartments and Protein Sorting (Sec 2 week 3)

Intracellular Compartment and percentage

Includes organelles and cytosol (which is half of the total cell volume)

Rough ER (job)

Membrane-bound ribosomes

Synthesis of soluble proteins and transmembrane proteins

Smooth ER (job - 2)

Phospholipid synthesis

detoxification

Organelle definition

discrete structure that is specialized to carry out a particular function (can either be membrane bound or not membrane bound)

Protein Sorting overview (3)

• Proteins are nuclear-encoded

• mRNA arrives in cytoplasm

• If it is cytosolic, no sorting signal, if needs to be somewhere specific it is given a signal sequence

Signal Sequence (2)

• Stretch of amino acids in a protein that direct to correct compartment

• Recognized by sorting receptors that bring proteins to their destination

2 ways of protein sorting

1. Post-translational sorting - Proteins are fully synthesized in the cytosol before sorting

2. Co-translational sorting - Proteins have an ER signal sequence and are sorted while being translated

Signal is usually on..

N-terminus

When do proteins need to be unfolded for transportation

When going into the mitochondria or chloroplast. They can remain folded for the nucleus and peroxisome

Who unfolds proteins entering the mitochondria

HSP70 chaperones

Method of sorting proteins to ER

• Protein being translated by ribosome with signal sequence is brought to ER

• Translation continues as protein is transfered in ER lumen through a protein translocator on ER membrane

• Hydrophobic signal sequence is cleaved, protein is released into ER lumen, translocon closes

Co-translation translocation of transmembrane protein

1. Also brought to ER, but instead of being released into ER lumen it is released into lipid bilayer

2. Hydrophobic Stop-transfer sequence stops translocation and becomes a-helix anchor in membrane

Start-transfer sequence

Internal signal sequence to start the translocation of a protein - similar to stop-transfer as it does not get removed like an n-terminal ER signal sequence but becomes a membrane spanning a-helix

Endomembrane system exchanges

Lipids and proteins.

3 pathways of endomembrane system

1. Secretory - ER to outside and ER to lysosome

2. Endocytic - Into cell from outside

3. Retrieval - Retrieval of Lipids/proteins for reuse

Exocytosis vs endocytosis

exo - vesicle contents delivered to extracellular space and vesicle becomes part of membrane

endo - Vesicle gets contents from extracellular space and the plasma membrane forms a vesicle

Secretory Pathway (2)

Constitutive - in all eukaryotes, continual delivery

Regulated - regulated secretion in specialized cells - eg pancreatic b-cells

Golgi Apparatus

Receives Proteins and lipids from ER and modifies them/dispatches them

Protein glycosylation

Starts in ER - the addition of carbohydrates to proteins - can also occur in golgi

Endocytic Pathway

• Endosomes fuse with each other

• move towards nucleus, matures through addition of hydrolases and H pump

• Eventually becomes lysosome

Lysosome digestion

Contains hydrolytic enzymes, low pH environment so it can digest products and release them into cytosol