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Lecture 17

BIOL 3113 Lecture Notes - Lecture 17: Endoplasmic Reticulum, Eukaryote, Golgi Cell


Department
BIOL
Course Code
BIOL 3113
Professor
Barbara S
Lecture
17

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17 Compartments of Eucaryotic Cells
Cells perform a variety of functions and carry out thousands of different reactions at the
same time
Strategies for organizing and isolating chemical reactions:
*aggregation of enzymes carrying sequences of reactions into large multiprotein
complexes (e.g. synthesis of proteins, DNA, RNA)
In both procaryotes and eucaryotes
*carrying different reactions and metabolic processes in membrane-bound compartments
most highly developed in eucaryotic cells
occupy ~half of the eucaryotic cell volume
many held in their locations by contact with the cytoskeletal proteins, especially with
microtubules
Major membrane-bound compartments of eucaryotic cells:
Cytosol
Nucleus (double membrane)
Endoplasmic reticulum
Golgi apparatus
Mitochondria (double membrane)
Chloroplasts (double membrane)
Lysosomes
Peroxisomes
Endosomes
Vesicles
Most organelles are formed from preexisting organelles which grow (by incorporation of
new molecules) and divide
Organelle growth requires:
Supply of new membrane lipids and proteins and soluble proteins to fill organelle interior
Proteins - continuously produced in cells - must be delivered to the appropriate
compartment
For some organelles - proteins delivered directly from the cytosol - for others - delivered
via the ER and Golgi

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Protein Sorting
Proteins - synthesized on ribosomes located in the cytosolic compartment. Once
synthesized the proteins must be sorted and transported into the compartment or location
where they will function
Three main mechanisms by which proteins are imported into organelles:
*through nuclear pores
cytosol nucleus
* by protein translocators - across membranes
cytosol ER; cytosol mitochondria
cytosol chloroplasts, cytosol peroxisomes
* by transport vesicles - from ER onward and between membrane-bound compartments
ER Golgi, Golgi lysosomes, Golgi vesicles, vesicles extracellular space
Sorting Signals
Direct proteins to the correct compartments
Proteins carry sorting signals in their amino acid sequence that determine into which
compartment they will be delivered - intracellular "zip" codes
Transport through nuclear pores (cytosol nucleus)
Nuclear Envelope
Double membrane that surrounds the nucleus and creates the nuclear compartment,
perforated by nuclear pores that allow two-way passage between cytosol and nuclear
compartment
Nuclear pores
Large complexes (~100 different proteins)
Regulate passage of molecules and particles into and out of the nucleus
Small water-soluble molecules pass freely through water-filled channels in the pore
Passage of larger molecules highly regulated - proteins to be transported must carry a
nuclear localization signal
Transport mediated by cytoplasmic proteins - nuclear transport receptors - that bind to the
recognition signal on the protein and interact with the nuclear pore fibrils
Transport requires GTP hydrolysis
Nuclear pores transport proteins in their fully folded conformation and ribosomal
components as assembled particles
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It is unknown how nuclear pore proteins handle transport of molecules in both directions
without collisions
Transport of Proteins into Mitochondria and Chloroplasts
Cytosolic proteins are translocated simultaneously across both outer and inner
membranes at specialized sites where the two membranes are in contact with each other
Transported proteins must be unfolded
Signal sequence is cleaved off after translocation
Chaperone proteins inside the mitochondria and chloroplasts help to pull proteins across
the membranes and help to refold after translocation
Further transport to a particular location inside the organelle requires additional sorting
signal in the protein (usually exposed after the first signal is removed)
Insertion of transmembrane proteins - requires multiple signal sequences in the protein
that start and stop the transfer across the membrane
Transfer of Proteins to the Endoplasmic Reticulum (ER)
ER is the most extensive system of membrane-bound compartments in eucaryotic cells
Two types of ER:
Smooth ER- lacks ribosomes
Rough ER- has attached ribosomes; site of protein synthesis
ER serves as an entry point for proteins destined for itself, for other organelles (Golgi,
endosomes, lysosomes) and for cell surface
Two kinds of proteins are transferred from the cytosol to the ER:
*Soluble proteins
Completely translocated across the membrane into the lumen
include: secretory proteins, Golgi proteins, lysosomal proteins
*Transmembrane proteins
translocated only partially across the membrane
include: plasma membrane proteins, ER membrane proteins, Golgi membrane proteins,
lysosomal membrane proteins
Directed to ER by an ER signal sequence
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