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Biology 2382B Lecture Notes - Oligosaccharide, Peroxisome, Organelle

Course Code
BIOL 2382B
Jessica Kelly

of 10
Lecture 4: Protein synthesis and transport
(The Endoplasmic Reticulum)
(Chp 13: p. 533-539, 549-554)
Protein Sorting/Targeting
Newly made peptides must be directed to the correct destination
Typical mammalian cell = 10,000 proteins Must be localized correctly to function
where they are supposed to
Targeting - Direct proteins to the right destinations (organelles) During or after
Sorting - Direct proteins to the secretory pathway (ER, Golgi, lysosomes)
Note: Though slightly different meaning sometimes used interchangeably…
General Principles of Protein Synthesis & Sorting
Many proteins are synthesized just by cytosolic ribosomes…
Those which remain in the cytosol
Those which are targeted to intracellular organelles such as (ER), mitochondria,
chloroplasts, peroxisomes, and nucleus (they have a specific signal sequence)
Other proteins are synthesized by ribosomes attached to ER (the rough ER)…
Those which reside in ER, secreted proteins, and proteins which are targeted to
PM, Golgi complex, and lysosomes
2 major protein-sorting pathways are known Non-secretory and Secretory
Where a protein will function in the cell will determine where it will be synthesize (i.e.
cytosol vs. ER rough)
Targeting sequence many found at the N-terminus (i.e. beginning of the
protein) will direct the protein to the ER where its synthesis will be competed
Secreted proteins cannot cross the hydrophobic cell membrane or organelle
Usually these targeted proteins must enter organelles through pores or channels
and often this occurs through ATP consumption (i.e. active transport)
ER Structure
Uninterrupted membranous tubules & vesicles
separated from cytoplasm cisterna which are
RER has ribosomes on the tubules (cisterna)
giving it the name “rough”
ER extend from nuclear membrane
Secretory Proteins Enter The ER
Bringing the previous techniques together we can see how microscopy allowed
visualization of organelles, homogenization and centrifugation helped purify
organelles such as microsomes and ER, and SDS-PAGE allows for newly translated
proteins to be identified
thus we can determine what kind of proteins are being
produced in the ER…
Before cell is lysed it is pulsed
addition of radioactive amino acids to
to cell’s environment cell will
uptake these and use them for protein
Protein translation is allowed to
continue for a period of time where the
radioactive amino acids are used in this
Disruption during synthesis results
in microsomes being created
Microsomes - small vesicles of RER
membrane with ribosomes attached that
are actively translation proteins