Lecture 9: Protein Sorting
Oct 8, 2013
movement of proteins -
within cell from different compartments,
out of cell,
or into cell
two types of protein sorting:
1. post-translational process: (protein fully synthesized in
cytosol before sorting) and then protein sorted to proper
organelle. protein can be maintained unfolded
plastids = chloroplasts
2. co-translational process:
proteins can already be folded.
proteins w Endoplasmic Reticulum (ER) signal sequence
associated with ER during protein synthesis
as protein synthesized, injected into ER (endoplasmic reticulum)
PROTEIN SORTING: MECHANISMS (3 types)
1. gated transport
movement of proteins btw cytosol and nucleus
nuclear pore complex is the gate that selectively choses which
proteins are allowed inside or out. pores in nuclear membrane -
does not allow passive transport in and out of nucleus. form
channels. allows for selective transport of macromolecules.
free diffusion of micro molecules (if smaller than 5 000
Daltons). move through nuclear pore complexes.
nuclear pore complex only selective for larger molecules bigger
than 5 000 Daltons.
proteins tend to be macromolecules.
nuclear import - movement of proteins through nuclear pore
complex from cytosol to nucleus
nuclear export - movement of proteins from nucleus to cytosol
nuclear envelope - lipid bilayer surrounding nucleus. not all
proteins allowed to move through NPC - only ones that have
NUCLEAR IMPORT -
proteins coming in are called cargo proteins. have specific
signal on them (nuclear localization signals). amino acid
sequence is NLS.
nuclear import receptor moves protein from cytosol to nucleus. it binds to amino acid sequence NLS. not specific sequence, but they
are rich in lysine and arginine.
once receptor binds to NLS, it binds to nucleoporins (are
proteins) in nuclear pore complex. can have different import
receptors that regulate movement of proteins. and receptors vary
because NLS sequences vary.
once cargo bound to receptor and receptor bound to nucleoporins,
then transported from nucleus to cytosol.
cargo proteins move out of nucleus have nuclear export signal
(usually hydrophobic amino acids). i.e.: ribosomes assembled in
nucleus have nuclear export signals. also, RNA molecules.
nuclear export receptor -
binds to Nuclear Export Signal
binds to nucleoporins in Nuclear Pore Complex
IMPORT AND EXPORT
maintained by plasma proteins called Ran GTPases. play role in
import and export.
RAN GTPase -
hydrolyzes GTP to GDP.
RAN GTPase cycles btw GTP-bound and GDP-bound states.
RAN-GAP regulates GTPase. it stimulates GTP hydrolysis by RAN-
Ran-GEF promotes exchange of GTP for GDP by Ran-GAP.
RAN-GAP localized in cytosol.
and RAN-GEF in nucleus.
high  RAN-GAP in cytosol therefore high RAN-GTP in nucleus
high  RAN-GEF in nucleus therefore low  Ran-GTP in cytosol
critical for direction of transport
Ran-GTP carries with it nuclear export receptors to cytosol
and RAN-GDP carries with it NTF2 to nucleus
NUCLEAR IMPORT OF CARGO
movement from cytosol to nucleus
nuclear import receptor binds to cargo in cytosol
receptor and cargo then move through nuclear pore complex into
Ran-GTP binding causes cargo release
now, you need to recycle import receptor.
empty import receptor and Ran-GTP move to cytosol.
Ran-Binding protein and Ran-GAP promote:
-GTP hydrolysis (GTP->GDP) -and release of empty import receptor
NUCLEAR EXPORT OF CARGO
nuclear export receptor -
binds to Ran-GTP and cargo in nucleus
receptor has NLS that allows it to bind
Ran-GTP predominantly found in nucleus
NTF2 moves Ran-GDP to nucleus
REGULATED TRANSPORT IN AND OUT OF NUCLEUS