Download protein targeting

Moving
Proteins Into membranes and Organelles
and
Vesicular Traffic
J a v a d
F a s a
J a m s h i d i
U n i v e r s i t y
o f
M e d i c a l
S c i e n c e s ,
N o v e m b e r
2 0 1 5
Protein Sorting
A typical mammalian cell contains up to 10,000 different
kinds of proteins
About half of the different kinds of proteins produced in a cell
are delivered to various membrane-bounded organelles
within the cell or to the cell surface
The delivery of newly synthesized proteins to their proper
cellular destinations, usually referred to as protein targeting
or protein sorting
Different kinds of Protein Sorting
Two very different kinds of processes:
Signal-based targeting
Vesicle-based trafficking
Signal-based Targeting
Targeting of a newly synthesized protein from the cytoplasm
to an intracellular organelle
Targeting can occur during translation or soon after synthesis
of the protein is complete
Proteins are sorted to the endoplasmic reticulum (ER),
mitochondria, chloroplasts, peroxisomes, and nucleus by this
general process
Vesicle-based Trafficking
Is known as the secretory pathway, and involves transport
of proteins from the ER to their final destination
For many proteins, including in the outside of the cell and
integral membrane proteins are transported to the Golgi,
lysosome, and plasma membrane by this process
The secretory pathway begins in the ER; thus all proteins
slated to enter the secretory pathway are initially targeted
to this organelle
Signal-based Targeting
How a given protein could be directed to only one
specific membrane??
How relatively large hydrophilic protein molecules
could be translocated across a hydrophobic
membrane without disrupting the bilayer??
Targeting Proteins to and Across the ER Membrane
All soluble proteins that will eventually be secreted from the
cell-as well as those destined for the lumen of the ER, Golgi, or
lysosomes-are initially delivered to the ER lumen
ER is the location where about one-third of the proteins in a
typical cell fold into their native conformations
Not all proteins that are targeted to the ER are actually
secreted from the cell
ER Signal Sequence
A 16- to 30-residue ER signal sequence in the nascent protein
directs the ribosome to the ER membrane
An ER signal sequence typically is located at the N-terminus
of the protein
For most secretory proteins, the signal sequence is cleaved
from the protein
Signal Recognition Particle (SRP)
A cytosolic ribnucleoprotein particle that transiently binds to
both the ER signal sequence and large ribosomal subunit
Targets the nascent protein-ribosome complex to the ER
membrane by binding to the SRP receptor on the membrane
The SRP is made up of six proteins bound to a 300-nucleotide
RNA, which acts as a scaffold for the hexamer.
Cotranslational translocation
Topological Classes of Integral Membrane Proteins
The topology ,number of times that its polypeptide spans
the membrane and the orientation of protein
The key elements of a protein that determine its topology
are membrane-spanning segments themselves, which
usually are a helices containing 20-25 hydrophobic
amino acids
Different Types of Transmembrane Proteins
Positioning type I single-pass proteins
Positioning type II single-pass proteins
Protein Modifications, Folding, in the ER
Membrane and soluble secretory proteins synthesized on the rough
ER undergo four principal modifications:
1. Covalent addition and processing of carbohydrates
(glycosylation) in the ER and Golgi
2. Formation of disulfide bonds in the ER,
3. Proper folding of polypeptide chains and assembly of
multisubunit proteins in the ER,
4. Specific proteolytic cleavages in the ER, Golgi, and secretory
vesicles.
Direct Proteins to the Mitochondrial Matrix
Amphipathic N-Terminal Signal Sequences Direct
Proteins to the Mitochondrial Matrix
located at the N-terminus, are usually 20-50 amino acids
in length.
Mitochondrial Protein Import Requires Outer-Membrane
Receptors and Translocons in Both Membranes
Nuclear Import/Export
Proteins imported to or exported from the nucleus contain
a specific amino acid sequence that functions as a nuclear
localization signal (NLS) or a nuclear-export signal (NES)
A cargo protein bearing an NES or NLS trans locates
through nuclear pores bound to its cognate nuclear
transport protein.
Nuclear Import
Vesicular Traffic
Secretory pathway and the mechanisms of vesicular traffic
that allow proteins to be secreted from the cell or delivered
to the plasma membrane and the lysosome.
The secretory pathway carries both soluble and membrane
proteins from the ER to their final destination
Transport of membrane and soluble proteins from one
membrane-bounded compartment to another is mediated
by transport vesicles
Vesicular transport
Protein Transport Through The Secretory Pathway
Mutants In 5 Stages In The Secretory Pathway
Different Coats in Vesicular Traffic
Different Coats in Vesicular Traffic
Mutants In 5 Stages In The Secretory Pathway
Forward and Retrieval Pathways