Biology 2382B Lecture Notes - Lecture 5: Secretion, Golgi Apparatus, Signal Peptide
31 Jan 2020
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SECTION 4
PROTEIN TARGETING/SORTING
− typical mammalian cell:
o 10,000 proteins
o function of protein likely depends on its correct localization → want the proteins to be
interacting properly with other proteins
− newly made peptides must be directed to the correct destination → the proteins have signal
sequences for this
− targeting → direct proteins to the right destinations (organelles)
o during or after synthesis
− sorting → direct proteins to the secretory pathway (ER, Golgi, lysosomes)
o **targeting and sorting are the same, but targeting is for organelles, and sorting is for
the secretory pathway
− proteins have signal sequences as part of their peptide chains which allow the cell to be able to
direct them to the right place in the cell
GENERAL PRINCIPLES OF TARGETING/SORTING
− many proteins are synthesized by cytosolic ribosomes
o some of these proteins remain in the cytosol
o some proteins are targeted to intracellular organelles such as, mitochondria, chloroplasts,
peroxisomes, and nucleus
▪ they have a specific amino acid signal sequence
▪ different for every organelle/part
− other proteins are synthesized by ribosomes attached to ER (the rough ER – RER)
− after these proteins are targeted to the ER, they might be sorted to Golgi complex, plasma
membrane, lysosomes, or even remain in the ER
− accordingly, two major protein-sorting pathways are known:
o non-secretory and secretory (organelles and ER/Golgi/PM)
− **important note: ribosomes are always in the cytoplasm
− mRNA made in the nucleus, and is then transferred into the cytoplasm, where translation starts
− at that point, the protein can stay in the cytosol and just be a cytosolic protein (no signal; not
targeted anywhere)
− it could also have a targeting signal such that the protein is taken to an organelle:
o mitochondria
o chloroplast
o peroxisome
o nucleus
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− or it could be target to the ER to enter the secretory pathway, where it could be sorted into:
o Golgi
o cell surface
o lysosome
− there are basic things that are needed for targeting:
o targeting sequence → tells the cell where the protein is going
o receptor for the signal sequence → allows the signal to be recognized
o source of energy and translocation channel → allows for the protein to get across
membranes
ER STRUCTURE
− uninterrupted membranous network of tubules and vesicles separated from the cytoplasm
− ER extends away from the nucleus
− the cisternae are stacked folds
o also present in the Golgi, but in the ER the cisternae are continuous with the nuclear
membrane (Golgi cisternae are not)
− may have ribosomes on the cisternae → RER (Golgi does not have ribosomes on it)
− ribosomes always in the cytoplasm
o the ER lumen does not contain ribosomes inside it
− ER lumen is different than the cytoplasm
o no ribosomes, which are always present in the cytoplasm
− being inside the ER lumen is similar to being extracellular, as the proteins inside it will be removed
from the cytoplasm → secreted to the outside, or be put on the cellular membrane
FUNCTION OF THE ER
How do we study the function of the ER?
− identify cellular features by microscopy isolate and homogenize them to free organelles
− sucrose density-gradient centrifugation of homogenate allows for isolation of microsomes &
ribosomes
− use isolated cellular organelles to reconstitute their function (protein translation?)
o can isolate different components and put them together in a controlled way to see how
they work
− SDS-PAGE is used to identify newly translated proteins
How do we know that secretory proteins come from the ER?
− do experiments to reconstitute parts of these components and see where the proteins are
− ex collagen → important for holding cells together, must be secreted
o take cells, lyse them with SDS and run them on gel → see that there is collagen inside the
cell
▪ where is that inside the cell?
