Protein sorting & trafficking Flashcards
What is the endomembrane system?
Series of compartments that work together to package, label, and ship proteins and molecules
What is the endomembrane system made up of?
Endoplasmic reticulum
Golgi apparatus
Why is important for proteins to be sorted into the right compartment?
Would result in chemical chaos
- degradative enzymes in lysosome needed separating from cytoplasm
After protein synthesis in the cytosol, where do proteins go?
> stay in cytosol > nucleus > ER > mitochondrion > peroxisomes > chloroplasts
What is the difference between targeting + trafficking?
Targeting = how proteins get to 1st destination
Trafficking = how proteins get to final destination
What are sorting signals?
Specific stretches of amino acids in proteins
- different organelles have diff sorting signals + are recognised by diff machineries
Which amino acids are important for nuclear targeting?
Where are they located?
Positively charged amino acids
= Lysine or Arginine
When protein is folded
–> stretches of amino acids situated on surface
How has the importance of +vely charged amino acids for nuclear targeting been proven?
Using T-antigen wild type and w/ an amino acid removed
Wild type efficiently sorted into nucleus
Mutated protein not targeted to nucleus
–> no longer contains nuclear import signal
What is special about nuclear membranes?
They’re contiguous w/ the endoplasmic reticulum
via continuous double membrane
What are nuclear pores composed of?
Large no. of distinct protein subunits
Fibrils protrude from both sides of complex
Define homomeric + heteromeric complexes
Homo = single type of subunit
Hetero = 2+ types of subunit
How do small + large molecules enter the nucleus?
Small = diffusion
Large = active transport through nuclear pores
What are NLS’s?
Nuclear localisation sequences
How are proteins imported through the nuclear pore complex?
- Nuclear import receptors recognise NLS’s on nuclear protein
- receptor-protein complex guided to pore by fibrils
- nucelar protein binds to pore –> opens
- active transport into nucleus w/ receptor using GTP
Describe the proteins imported into nuclei
Fully synthesised
Fully folded
How are mitochondrial targeting sequences different to nuclear ones?
> always at N-terminus
> forms amphipathic alpha-helix
–> +ve charge lies on 1 face of helix
Briefly, how do proteins get into the mitochondrial matrix?
- Receptors on outer membrane recognise targeting sequences
2. Conducting channels allow proteins to cross membranes
What are the 2 conducting channels?
TOM = translocator of outer membrane
TIM = translocator of inner membrane
In what form must proteins be in order to be imported into mitochondria?
Unfolded
How are mitochondrial proteins prevented from folding?
By cytoplasmic chaperone proteins
Why is mitochondrial protein targeting complex?
> Proteins can be outer membrane, inner membrane, inter-membrane space or matrix
have their own genome + protein synthesis machinery
How do nuclear import receptors exit the nucleus?
Ran-GTP binds to receptors inside nucleus
Ran-GDP + Pi dissociate in cytosol
What is required for the translocation of mitochondrial proteins across the inner membrane?
Membrane potential (-ve inside)
Energy from ATP hydrolyse
