Lecture 16 - Protein sorting at the cytoplasm Flashcards
How does the cell know where a protein is supposed to go?
linear signal sequences are the sorting signals for protein translocation into organelles
Nuclear envelope
consists of 2 concentric membranes that are perforated by nuclear pore complexes
Nuclear pore complexes
perforate the nuclear envelope in all eukaryotes
- composed of roughly 30 different proteins, or nucleoporins (mediate the passage of molecules)
Mesh of the nuclear pore complex
formed by unstructured proteins; acts as a sieve that restricts the diffusion of large macromolecules while allowing smaller molecules to pass
Nuclear localization sequence (NLS)
proteins with this sequence are directed into the nucleus
- nuclear localization signals have flanking basic clusters
What sequence do proteins need to leave the nucleus?
a nuclear export sequence (NES)
Import receptors (importins)
soluble cytosolic proteins that contain multiple low affinity binding sites for the FG repeats found in the unstructured domains of several nucleoporins
- they bind to proteins with NLS and mediate passage into nucleus
What does the importin-cargo complex do?
it locally dissolves the gel-like mesh and can diffuse into and within the NPC pore
What is the role of Ran?
the concentration of Ran bound to GTP provides energy and directionality
- for importins, Ran-GTP in the nucelus promotes cargo dissociation
Exportins
NES receptors
- Ran-GTP promotes cargo BINDING
GAP and GEF
GAP = GTPase-activating protein
- triggers GTP hydrolysis (GTP => GDP)
- cytosolic
GEF = guanine nucleotide exchange factor
- promotes the exchange of GDP for GTP
- nuclear
Alpha and beta subunits of importins
alpha: binds the nuclear localization signal (NLS)
- no NLS => alpha subunit cannot bind => protein stays in cytoplasm
beta: binds the unstructured chains
How does the importin-beta carry cargo inside?
in the cytoplasm a GTP molecule is hydrolyzed and Ran dissociates (releasing cargo molecule)
What do exportins bind to?
- the export signal (on cargo) either directly or via an adaptor
- NPC proteins to guide their cargo into the cytosol
Can proteins contain both NESs and NLSs?
yes!
proteins can constantly be shuttled between the nucleus and the cytosol
Where are most mitochondrial proteins synthesized?
in the cytoplasm and then translocated into the mitochondria
Do mitochondrial and plastid proteins have signal sequences?
yes; they must be located at the N-terminus part b/c the pores of mitochondria and plastids are small so only unfolded proteins can fit through
TIM & TOM systems
move polypeptide chains through the 2 membranes of the mitochondria
TOM = translocase of the outer membrane
TIM = translocase of the inner membrane
TIC and TOC for chloroplasts
Hsp-70 chaperones
interact with mitochondrial proteins at the cytoplasm and prevent them from folding and aggregating
