BIOLOGY 2D03 Lecture Notes - Surface 2, Melanosome, Lysosomal Storage Disease

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28 Jan 2013
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EXOCYTOSIS
Signals on proteins are the key to targeting
Bound ribosomes make ALL secretory and membrane proteins
Pulse chase to figure out vectorial transport (in pancreas)
Vesicular Transport:
reduces need to cross membranes
allows for compartmentalization
problems:
targeting
maintaining compartmental identity (keep certain proteins/membranes in a
given compartment)
ER:
cytoplasm-filling reticular network
prominent in secretory cells and steroid metabolizing cells
continuous with nuclear envelope
MORPHOLOGY -
cisternae = flat sacs that makeup the ER
50% of total cell membrane
lumen:
separate secretory from cytoplasmic proteins
prepare proteins for export
ion storage
FUNCTIONS -
lipid synth, ion storage and trans, steroid metabolism
RER only: export of proteins to secretion, membrane, lysosome, endosome
ER can be isolated by centrifugation
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BLOBEL – “signal hypothesis”
signal sequence to translocate through membrane and is later cleaved
sorting begins at the ribosome (moves to ER during translation)
SRP – signal recognition particle **adaptor molecule**
free floating and binds signal sequence
recognized by and binds to ER membrane receptor
causes a pause in translation until the pore is reached
translation is co-translational (otherwise signal sequence is hidden)
SIGNAL SEQ – not highly conserved
general features:
hydrophobic core
cleavage site – small hydrophobics
**not all are cleaved**
SEC61 TRANSLOCON – forms pore
tightly regulated – mech. unknown
pore lines up perfectly with ribosome exit tunnel
ASSESSING TRANSLOCATION EXPRIMENTALLY:
the membrane is impermeable to trypsin
find that translocated proteins are proteolytically cleaved
can also see co-translation
INTEGRAL MEMBRANE PROTEIN TRANLOCATION
“stop transfer sequences” – stop the translocation but not translation
hydrophobic sequences that stick into the membrane
channel exit mech. still uncertain
signal sequence orientation determines whether N or C terminus is outside
many start-stops polytopic membrane proteins
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