Topic 4A - Protein Transport Across Membranes Flashcards
which organelles are completed isolated from vesicular traffic?
mitochondria and chloroplasts
the nuclear pore complex allows transport between which two areas of the cell?
nucleus and cytosol
is NPC travel bidirectional or unidirectional?
bidirectional. proteins synthesized in cytosol and transported to nucleus…
what is a protein signal sequence? and where must it located on protein travelling through NPC?
it is amino acid sequences that tell the protein where to go/ it is recognized by sorting receptors.
can be anywhere in amino acid sequence as long as on the protein’s surface for the NPC
what travels in and out of the NPC?
in: histones, DNA, RNA, enzymes
out: mRNA, tRNA, newly formed ribosome subunits
which aspect of the NPC structure helps block passive diffusion of macromolecules
tangled mesh of disordered domains in central pore
what can diffuse through the NPC?
small water-soluble molecules can travel through the aqueous passage.
what helps cargo proteins enter/ exit NPC?
nuclear import/ export receptors AND nuclear import/ export adaptor proteins
they bind nuclear localization signals/ the associated cargo protein
is travel through NPC active or passive?
active!
how do proteins travel in and out of nuclear pore?
bind to nuclear import/ export receptors which bind and dissociate to domains of channel nuclear porins
hop from one nuclear pore to another (possible because of FG repeats) until they reach the cytosol/ nucleus and unload the cargo
import receptors then hop back to their origin
how is directionality of NPC imposed?
by Ran GTPase!
Ran-GTP/Ran-GEF localized in nucleus
Ran-GAP/ RAN-GDP localized in cytosol
process of Ran-GTPase and cargo unloading/loading (for nuclear import)
cargo protein and nuclear import receptor bind in cytosol
Ran-GTP binds to nuclear import receptor as it enters the nucleus –> cargo released
nuclear import receptor hops back to cytosol with GTP and then Ran-GTP converted to Ran-GDP (with help of Ran-GAP)
nuclear import receptor releases Ran-GDP with help from RAN-GEF
nuclear import receptor free to bind next cargo protein
difference between import/ export in regards to cargo binding
import: can bind either cargo or Ran
export: can bind Ran-GTP and cargo at same time or nothing
example of feedback regulation in NPC
SREBP controls expression of cholesterol
it is anchored in ER membrane as long as enough cholesterol is interacting with SCAP
if not enough, SCAP is empty and gets packed with SREBP into transport vesicles to Golgi
here, SREBP gets cleaved –> its cytosolic tail moves to nucleus where it binds to promoters of genes that encode proteins involved in cholesterol biosynthesis
how is the NPC regulated?
turn import/ export signals on/ off through phosphorylation
