lecture 3 Flashcards
intracellular compartments
mitochondria, golgi apparatus, ER, nucleus, plasma membrane, free ribosomes, peroxisome, cytosol, lysosome, endosome
volumes will differ from cell to cell
half the volume of cells is cytosol
rough er
membrane-bound ribosomes
synthesis of soluble proteins and transmembrane proteins for the endomembrane
smooth ER
phospholipid synthesis, detoxification
organelles
a discrete structure or subcompartment of a eukaryotic cell that is specialized to carry out a particular function
protein sorting
proteins are nuclear encoded
mRNA arrived in the cytoplasm (translatioon starts on ribosomes in cytosol)
cytosolic proteins dont have a sorting signal their default location is the cytosol
proteins that are sorted have a sorting signal called signal sequence
signal sequence
stretch of amino acid sequence in a protein
specifies where protein will go in the cell
signal sequences are recognized by sorting receptors
post-translational sorting
proteins are nuclear encoded, they are fully synthesizied in cytosol before sorting
when folded:nucleus, peroxisomes
- targeted by a signal sequence for import into nucleus/peroxisomes
when unfolded: mitochondria, plastids
co-translational sorting
proteins are nuclear-encoded
they have ER signal sequence
associated with ER during protein synthesis in the cytosol
sorting proteins to peroxisomes
containes enzymes for oxidative reactions
enzymes imported in through the transmembrane protein complex
sorting proteins to mitochondria/chloroplasts
have their own genomes and ribosomes
most proteins for these cells are nuclear-encoded
they are translated into the cytosol and targeted by a signal sequence for import
proteins are UNFOLDED for import
sorting proteins to the ER
co translational sorting
proteins are nuclear encoded
proteins have an ER sequence
ER signal sequence is hydrophobic
proteins that enter the ER are soluble proteins and transmembrane proteins
cotranslational translocation: soluble protein
- translation starts, N-terminal ER signal sequence emerges
- recognized by SRP, elongation arrest by SRP
- SRP-ribosome complex> SRP receptor> translocon
- transolocon opens
- protein synthesis resumes with protein transfer into ER lumen
- signal peptidase cleaves ER signal sequence
- protein is released into ER lumen
- translocon closes
destination: lumen of endomembrane of organemme or secretion at PM
cotranslational translocation: transmembrane protein
1-5 same
6. stop transfer sequence enters translocon (internal hydrophobic segment)
7. protein transfer stops and transmembrane domain is released into lipid bilayer
8. signal peptidase cleaves ER signal sequence and translocation closes
9. protein is completed
destination: membrane of an endomembrane organelle or in the plasma membrane
ER signal sequences
N-terminal ER signal sequence
at N-terminus of protein, stretch of hydrophobic amino acids, removed by signal peptidase
internal ER sequence
start-transfer sequence, stretch of hydrophobic amino acids, NOT removed, they remain part of the protein
endomembrane system
ER, golgi, endosomes, lysosomes
secretory pathway
-proteins and lipids are made in ER
delivered to other compartments
ER>outside is exocytosis
ER>lysosomes via endosomes
endocytic pathway
contents move into cell
endocytosis
retrieval pathway
retrieval of lipids, selected proteins for reuse
exocytosis
vesicle conetents are delivered to extracellular space
vesicle membrane becomes part of pm
Endocytosis
vesicle luminal contents come from extracellular space
plasma membrane forms vesicle membrane
vesicle
small, membrane-enclosed organelle in cytoplasm of eukaryotic cell
shuttles components back and forth in the endomembrane system
maintenance of membrane protein asymmetry
each membrane protein has a specific oreintation, resule of insertation in ER
protein assymmetry is maintaied through vesicular transport
golgi
recieves proteins and lipids from the ER, modifies them and then dispatched them to other destinations in the cell
protein glycosylation
starts in the ER, single type of oligosaccharide is attatched to many proteins
complex oligosaccharide processing occirs in golgi
endosomes
membrane bound organelles
contain material ingested by endocytosisl
lysosomes
membrane bound organelles, contain hydrolytic enzymes to digest worn-out proteins, organelles, other waste
main site of intracellular digestion
