J a v a d J a m s h i d i

F a s a 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

Proteins Into membranes and Organelles andVesicular Traffic

Moving

The Cytoskeleton Functions

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 within membrane-enclosed vesicles

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

Contain one or more positively charged amino acids adjacent to a continuous stretch of 6-12 hydrophobic residues (known as the hydrophobic core-essential for their function)

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

Targeting of Peroxysomal Proteins

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 TrafficSecretory 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

Protein Transport Through The Secretory Pathway

Mutants In 5 Stages In The Secretory Pathway

Mutants In 5 Stages In The Secretory Pathway

Different Coats in Vesicular Traffic

Different Coats in Vesicular Traffic

Forward and Retrieval Pathways