Protein Movement between Compartments * Most proteins are synthesized on cytoplasmic ribosomes and must be delivered to their ultimate compartment of residence. * Proteins contain sorting signals that direct their movement throughout the cell. * These sorting signals are recognized by specific receptors that mediate delivery to the appropriate organelle. * There are three major types of protein traffic between compartments: 1) Gated transport 2) Transmembrane translocation 3) Vesicular transport
Transmembrane Translocation * Proteins are directly translocated across the membrane bilayer. * Translocation is performed by a membrane protein complex that forms a translocation pore. * Proteins pass through the membrane bilayer as unfolded chains. * Two major strategies are used to accomplish this feat: co-translational & post-translational import.
Figure 12-7 Vesicular Transport
Protein sorting signals
Signal sequences direct protein delivery SS Destination Mitochondria Cytoplasm 1. Deletion of signal sequence (SS) SS 2. Addition of a signal sequence Mitochondria Cytoplasm
Mitochondrial Protein Import * Mitochondria utilize the energy from electron transport and oxidative phosphorylation to synthesize the majority of the cell's ATP. * Most mitochondrial proteins are synthesized on cytoplasmic ribosomes and are post-translationally imported into this organelle. * Because of the double membrane surrounding this organelle, there are four targets for mitochondrial proteins: 1. Outer membrane3. Inner membrane 2. Intermembrane space4. Matrix space * Mitochondrial proteins usually contain an N-terminal targeting sequence that is capable of forming an amphipathic -helix; positively-charged residues are clustered on one side of the helix and uncharged residues are present on the other. * The mitochondrial outer membrane contains specific receptor proteins that bind to the mitochondrial targeting signal.
Figure * Signal sequence for mitochondrial protein import. * Note the amphipathic nature of the -helix.
Figure Protein translocators in mitochondrial membranes (Matrix/Inner Membrane) (Inner Membrane)
Mitochondrial Protein Import (cont’d) * Translocation into the mitochondrial matrix requires both ATP hydrolysis and an electrochemical gradient across the inner mitochondrial membrane. * Translocation occurs at sites where the inner and outer membrane are in close apposition. These regions are known as contact sites. * Proteins are imported into the mitochondria in an unfolded state. * Maintenance in an unfolded state is mediated by hsp70 proteins that act as molecular chaperones. * Protein transport into the inner membrane or intermembrane space requires additional targeting signals. * Much of our current knowledge of mitochondrial protein import has come from in vitro studies with isolated mitochondria.
Studying mitochondrial protein import in vitro IMPORT ? * Isolated mitochondria are mixed with the radioactively- labeled protein to be studied
* Import may be detected by one of the following methods: 1) Density gradient centrifugation; if imported, proteins will fractionate with the organelle. 2) SDS-PAGE analysis to determine if the signal sequence was removed during the import reaction. 3) Protease protection assays; imported protein will be protected from the action of added proteases. * By adding or removing different components from the import reaction, one can determine the requirements for protein import. IMPORT ?
Proteins enter into the secretory pathway at the ER where they are co-translationally inserted into the ER membrane. Proteins then travel to successive organelles via membrane-bound intermediates. Secretory Pathway
Functions of the ER * The entry point for proteins that proceed through the secretory pathway. * Modification of proteins: a predominant modification is the glycosylation of specific asparagine residues (N-linked sugars). * Quality control: proteins must be properly folded before they are allowed to leave the ER. Proteins that fail to achieve a native state are degraded. * Sequestration of Ca ++ from the cytoplasm. * Primary site of lipid biosynthesis.
Figure The Signal Hypothesis George Palade: 1974 Nobel Prize in Medicine
Protein Import into the ER Step 1: Establishing a tight interaction with the ER membrane * A hydrophobic signal peptide, usually at the N-terminus of the protein, directs entry into the ER. * The signal peptide is recognized by the Signal Recognition Particle (SRP) as soon as it emerges from the ribosome. This interaction arrests translation. * The ER membrane contains an SRP receptor that mediates the initial association of the SRP-ribosome complex with the cytoplasmic face of the ER. * The ribosome subsequently associates with a translocation complex (the Sec61 complex) in the ER membrane and the SRP is released back into the cytosol.
Protein Import into the ER Step 2: Co-translational translocation of the polypeptide * Upon association with the ER, the ribosome resumes translation and co-translationally inserts the polypeptide chain into the ER lumen through a translocation pore. * The protein is passed through the membrane as a single, unfolded chain and folds into its native conformation within the ER lumen. This folding process requires protein chaperones. * The N-terminal signal peptide is removed by Signal Peptidase, a protease present in the lumen of the ER. * Integral membrane proteins contain "stop transfer" sequences that result in a block to the translocation process.
Panel 12-1 Genetic approaches for studying the mechanism of protein translocation Wild-type Enzyme in cytosol: cell lives without histidine Engineered Cell Enzyme targeted to ER: cell dies without histidine Mutant Engineered Cell Not all enzyme targeted to ER: cell lives without histidine
Figure Synthesis of the lipid- linked precursor oligosaccharide in the rough ER membrane
Possible functions for the N-linked oligosaccharide chains? 1.Promoting protein folding & stability. 2.Protecting the protein from proteolysis. 3.Serving as a targeting determinant. 4.Facilitating or directing anterograde (forward) transport. 5.Promoting cell-to-cell adhesion.
Figure The role of N-linked glycosylation in ER protein folding
Figure * The ER functions as a quality control organelle. * Proteins that are not properly folded are exported from the ER and degraded in the cytosol. The export & degradation of misfolded ER proteins
Quality control in the ER and Cystic Fibrosis A particular deletion that removes three nucleotides in the Cftr gene is the most common mutation responsible for this disease. This deletion results in the removal of a phenylalanine residue, F508. The encoded protein is recognized by the ER quality-control machinery and is ultimately targeted for degradation (in the cytoplasm). However, the encoded protein would be FUNCTIONAL if it was allowed to go to the plasma membrane. * * * * Knowing the above, how might you try to treat CF patients that possess this cftr allele? M * Degradation Plasma membrane
Lipid synthesis in the ER * The cytoplasmic half of the ER bilayer is the primary site of phospholipid synthesis. The enzymes that catalyze these reactions are ER membrane proteins whose active sites face the cytosol. * Phospholipid translocators function to "flip" specific phospholipids from one half of the bilayer to the other. * Specific phospholipid transfer proteins (PLTPs) transport phospholipids from the ER to mitochondria and peroxisomes.
Vesicular Transport * The lumen of each compartment communicating by way of vesicular traffic is topologically equivalent. * The two primary pathways for vesicular traffic are known as the biosynthetic-secretory and the endocytic pathways. * A transport vesicle must select the cargo to be transported to the next compartment and exclude that which is to remain behind. * To ensure compartment identity, a transport vesicle must fuse only with the appropriate target organelle.
Protein coats facilitate multiple steps of vesicular transport
Coated Vesicles * Most transport vesicles form from specialized "coated" regions of the membrane and bud off as coated vesicles. * These coats are protein structures that form on the cytosolic face of a membrane region that will form the transport vesicle. * Several coat structures have been identified in eukaryotic cells and each appears to perform a distinct transport function. * Coated vesicles generally mediate the directional flow of specific types of membranes. * The assembly of a coat structure on a membrane may be the driving force in bud formation. * Coat proteins play an important role in the selection of vesicle cargo.
Generation of membrane curvature a.Membrane deformation by proteins that exert mechanical force. b.Curvature generation by scaffolding proteins (coat proteins). c.Curvature generation by a hydrophobic insertion (wedging) mechanism.
Clathrin-coated Vesicles * The primary component of one membrane coat is clathrin, a large protein complex composed of three subunits each of a heavy chain and a light chain. * Clathrin-coated vesicles mediate Golgi-to-lysosome protein delivery and plasma membrane receptor- mediated endocytosis. * Clathrin coat assembly provides the mechanical force necessary for bud emergence and vesicle formation. * Other proteins in this coat, known as adaptins, mediate the binding and sequestration of specific transmembrane receptors and their bound cargo.
Figure Formation of COPII-coated vesicles Sec13/31 “cage”
How does a transport vesicle find its correct destination?
Vesicle Targeting * To ensure compartment identity, transport vesicles must fuse only with the appropriate target membrane. * This specificity is mediated by two classes of proteins: the Rab family of monomeric GTPases and the transmembrane SNARE proteins that mediate membrane fusion. * The active GTP-bound Rab proteins interact with a diverse set of Ras effector proteins that mediate vesicle transport, tethering and fusion to the appropriate target membrane. * Membrane fusion is facilitated by the pairing of complementary transmembrane receptors present on the vesicle (v-SNAREs) and the target membrane (t-SNAREs). * SNARE complex disassembly after membrane fusion is catalyzed by NSF, a cytoplasmic ATPase.