Cell Bio Exam 3 Flashcards
3 major pathways in eukaryotic cell for protein trafficking: all from cytoplasm to destination
Transport through nuclear pores (to nucleus)
Transport across membranes to chloroplast, mitochondria & peroxisomes
Transport by vesicles from ER to Golgi
How do the proteins know where they’re going?
SORTING SIGNALS aka SIGNAL SEQUENCES
Where do sorting signals aka signal sequences occur?
Occurs in the primary & secondary structures of polypeptides
What is a common required feature of sorting and trafficking processes to move proteins across the membrane to deform & separate or fuse membranes?
They require ENERGY input
What if a protein doesn’t have a protein signal?
By default, remain in the cytosol
Cytosol
contains many metabolic pathways
protein synthesis
cytoskeleton
Nucleus
contains main genome
DNA and RNA synthesis
ER
synthesis of most lipids
synthesis of proteins for distribution to many organelles and to the plasma membrane
Golgi apparatus
modification, sorting, and packaging of proteins and lipids for either secretion or delivery to another organelle
Lysosomes
intracellular degradation
Endosomes
sorting of endocytosed material
Mitochondria
ATP synthesis by oxidative phosphorylation
Chloroplasts
ATP synthesis and carbon fixation by photosynthesis
Peroxisomes
oxidation of toxic molecules
What are the 2 suspected origins for the evolution of internal membrane compartments?
Plasma membrane invaginations: endomembrane system
Endosymbionts: mitochondria & chloroplasts
Double membrane & own genome
Anaerobic pre-eukaryotic cell→ early aerobic eukaryotic cell
Endosymbiosis theory is supported by several observations
Organelles have circular chromosomes (like bacteria)
Organelle genes are more similar to bacterial genes than to those found within the nucleus
During the evolution of mitochondria and chloroplasts, most genes have been lost or transferred to the nucleus
Nuclear Transport Steps
Nuclear pore complexes: allows proteins & mRNAs to traffic across the nuclear membrane
Nuclear localization signals (NLS): “nuclear” proteins associate with NLS receptors which MOVE THE CARGO ACROSS the nuclear membrane in GTP dependent manner
Nuclear export signals (NES)
BASIC AMINO ACIDS
lysine & arginine
NLS (Nuclear localization signals) receptors use what?
GTP hydrolysis
Mitochondria & Chloroplast Import
Proteins have signal sequence (ss)
Organelles membrane localized RECEPTORS
Transmembrane TRANSLOCATOR COMPLEX
How are proteins transferred across the mitochondrial membrane?
UNFOLDED
How do the signal sequences get removed in Mitochondria & chloroplast import?
SIGNAL PEPTIDASES - this is unlike nuclear localization where shuttling can occur
What kind of alpha helix does the mitochondrial signal sequence have?
An amphipathic (polar & nonpolar)
ER import vesicle traffic
Signal receptor recognition particle (SRP), SRP receptor
ER: first decision point for protein destinations
Vesicle traffic: coats & SNARES
Glogi: the major destination point for paths→”bus station”
Endosome = the other major: “bus station”
How are the proteins that are being made directed for translocation into the ER?
By SIGNAL RECOGNITION PARTICLE (SRP) and the SRP receptor & translocation channel
What are the first two steps of ER import?
- Cotranslational insertion of signal sequence bearing proteins & into the ER membrane
- SRPs associates with the SRP associated with the nascent ss, this stalls the ribosome and aids in docking on the ER membrane at the SRP receptor
How are proteins modified in the ER & golgi?
Through glycosylation
Glycosylation
Where a carbohydrate to a macromolecule, such as a protein & lipids
Glycosylation is done by oligosaccharide protein transferases by a HIGH energy bond
Disulfide bond formation is also catalyzed in the ——
ER
What is the function of Chaperone proteins?
They make sure the proteins are properly folded and inserted in the membrane
What is the orientation of plasma membrane proteins dependent on?
On their synthesis & trafficking history
PROPER INSERTION OF THESE PROTEINS IS ESSENTIAL FOR THEIR FUNCTION
Which terminal signal sequence in many secretory proteins cleaved and how are they cleaved?
N-terminal are cleaved off by a signal peptidase
Stop transfer & stop transfer sequences inserts what into the ER membrane for single and multipass transmembrane domain proteins?
Hydrophobic transmembrane helices
Name an example of a multipass ™ (transmembrane) protein
bacteriorhodopsin
The unfolded protein response (UPR) pathway
is an example of a signal pathway that helps cells maintain quality control over protein folding & biogenesis by INCREASING chaperone activities
Name the disease that is caused by the fact that quality control at the ER chaperones is extremely stringent (tight)
cystic fibrosis (CF)
CF variant proteins can be slightly misfolded & misfolded proteins are rapidly degraded regardless of whether they are at all functional
How are vesicles made and how do they know where to go?
Vesicle transport
Vesicles move soluble and membrane-associated proteins between cellular compartments
Secretory pathway
“Forward” flow: ER→Golgi→Plasma Membrane→Lysosome
“Retrograde” flow: Golgi→ER “retrieval”
Endocytic pathway
Flow: plasma membrane→endosome→lysosome→plasma membrane
How are vesicles formed and targeted?
By the action of protein coats
Protein Coats
Coats serve to deform membranes so that they can be “pinched off” by dynamin: GTP hydrolysis
What is responsible for targeting & fusion specificity of vesicles to their destination membrane?
Rabs, tethers, v-SNAREs (v for vesicle) & t-SNAREs (target found on membrane)
Rabs recognize tethers, then v-SNAREs & t-SNAREs drive the fusion reaction & help provide membrane specificity
What is the key sorting station as is the endosome?
TRANS face of the golgi
2 golgi derived pathways to the plasma membrane
constitutive & regulated