Lecture 11&12 - Membrane bound organelles and Protein sorting Flashcards
internal organelle evolution book section
One suggested pathway for the evolution of the eukaryotic cell and its internal membranes As discussed in Chapter 1, there is evidence that the nuclear genome of a eukaryotic cell evolved from an ancient archeaon. For example, clear homologs of actin, tubulin, histones, and the nuclear DNA replication system are found in archaea, but not in bacteria. Thus, it is now thought that the first eukaryotic cells arose when an ancient anaerobic archaeon joined forces with an aerobic bacterium roughly 1.6 billion years ago. As indicated, the nuclear envelope may have originated from an invagination of the plasma membrane of this ancient archaeon—an invagination that protected its chromosome while still allowing access of the DNA to the cytosol (as required for DNA to direct protein synthesis). This envelope may have later pinched off completely from the plasma membrane, so as to produce a separate nuclear compartment surrounded by a double membrane. Because this double membrane is penetrated by nuclear pore complexes, the nuclear compartment is topologically equivalent to the cytosol. In contrast, the lumen of the ER is continuous with the space between the inner and outer nuclear membranes, and it is topologically equivalent to the extracellular space
(slide 4)
What is the function of peroxisomes
Fatty acid beta oxidation
WHAT IS IN THE ENDOMEMBRANE SYSTEM
Endoplasmic reticulum, lysosomes, Golgi, Secretory vesicle
What is the benefit of an endomembrane system
molecules can get from one to the other without having to cross a membrane
How are organelles in the endomembrane system connected
protein trafficking system
what is the entry point for newly synthesised proteins in the endomembrane system
Endoplasmic reticulum
What is a translicon
Protein conducting channel
How long are ribosomes on the RER
Until protein has been synthesised, then dissociate into big and small subunits and search for mRNA
Where does N-linked glycosylation occur
ER
What is N-linked Glycosylation + mechanism
the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom
What is the most important organelle in protein synthesis
ER - mis-folded or mis-assembled dont leave ER - folding and quality control - disulphide bond formation
Calnexin binding
Calnexin binds to glycans found in membrane proximal domains while the soluble calreticulin associates with glycans within the ER lumen
What is a reducing environment
An area where few disulphide bonds form
Describe the Golgi network
Slide 11
O linked glycosylation
O-linked glycosylation is the attachment of a sugar molecule to the oxygen atom of serine (Ser) or threonine (Thr) residues in a protein. O-glycosylation is a post-translational modification that occurs after the protein has been synthesised
Why are lysosomes degradative organelles
contain enzymes called acid hydrolases, work at lower pHs
How many different protein molecules are in a eukaryotic cell
10 billion
Where does protein sorting start?
Cytosol
Where does the sorting signalling tend to be to import into mitochondria
N-terminus of proteins (amphipathic)
What key do proteins that are imported into the nucleus (signalling) have
Patches of positively charged amino acids
What is nuclear targeting
Proteins are directed into the nucleus using patches of positively charged amino acids
Why is the nuclear membrane described as contiguous
Envelope - two membranes, and continuous with Endoplasmic reticulum
What is the function of nuclear pores
All movement of molecules into and out of the nucleus occurs through nuclear pores
(slide 26 - nuclear pore complex)
How does protein import through nuclear pore complex occur
Nuclear import receptors recognise NLS’ on prospective nuclear protein
Complex of receptor and protein with NLS guided to nuclear pore by fibrils
Binding of nuclear protein to pore opens pore
Active transport into nucleus (together with receptor
What do GTPases do
Work as molecular switches - work in GTP bound state or GDP bound state in different conformations
How does GTP convert to GDP and reverse
Hydrolysis eg. Ran GAP (GTPase activating protein) activates Ran-GTP to hydrolyse to Ran-GDP
(reverse Ran-GEF - Guanine nucleotide Exchange Factor) facilitates the exchange of GDP (removed completely) for GTP
Where are Ran-GEFs located
Nucleus
Where are Ran-GAPs located
Cytoplasm
Watch animation 15.1
Nuclear import/export (slide 13)
What are some key features of mitochondrial targeting sequences
Always at the N-terminus
Varies in size from 20-80 amino acids
Multiple positively charged amino acids make an amphipathic a-helix
(Watch Movie 15.2)
How do proteins get into mitochondria (channels)
Enter TOM and TIM (translocation channels - extremely specific) (Outer and inner membranes)
How do proteins enter channels
proteins need to be unfolded proteins and associate to chaperone proteins to keep them unfolded to be imported (active process as well as gradient creation)
Why are mitochondria negatively charged
Pumping out protons in generation of ATP
Where are mitochondrial proteins located
Outer membrane, Inner membrane, Inter-membrane space or Matrix (own genome, and protein synthesis machinery)
How does a protein target to the ER
Hydrophobic signal sequence (mostly at N terminus)
Describe ER targeting and translocation
Ribosome binds to RNA causing translation
Binds to SRP (Signal Recognition Particle) to signal peptide causes a pause in translation
SRP-bound ribosome attaches to SRP receptor in ER membrane,
Translation continues and translocation begins
SRP and SRP receptor displaced and recycled
Pushing of protein into lumen of ER, negatively charged n-terminus pushed through translicon - cotranslational translocation
Protein refolds once it has entered endoplasmic reticulum
(movie 15.4 and YT link slide 21)
Ribosome-nascent chain complex
