Membrane Trafficking PDF

Membrane trafficking    **MLO8.**  Explain, with the aid of diagrams, the key organelles within a cell and how they function.\      **MLO11.**  Describe how different cellular compartments are generated, maintained and communicate with each other. Why we compartmentalize cells: - Increase in se...

Membrane trafficking    **MLO8.**  Explain, with the aid of diagrams, the key organelles within a cell and how they function.\      **MLO11.**  Describe how different cellular compartments are generated, maintained and communicate with each other. Why we compartmentalize cells: - Increase in self complexity - Increase in size - Segregation allows different micro environments to be established, making it more efficient - Increased surface area for reactions to occur - Decreases the distance molecules have to travel Endomembrane system: - Nuclear membrane - ER - Golgi body - Peroxisomes - Endosomes - Lysosomes All the compartments communicate via vesicles and are derived from the invagination of the plasma membrane. These compartments tend to be more oxidising environments than other parts of the cytosol How proteins move to the correct compartment: - Transportation through a membrane pore - Transported across membrane bilayer - Transported via vesicles Sorting signals: - Specific sequences of amino acids between 50-60 amino acids long which target protein to a specific organelle - Signal sync sequences are usually cleaved, described as being always necessary and sufficient to direct a particular protein to an organelle - In the absence of a sorting signal, a protein is targeted to the cytosol Nuclear membrane: - Nuclear envelope has a double membrane: inner membrane is responsible for finding nuclear proteins, outer membrane resembles and is continuous with the ER - Nuclear pores: cytosolic fibrils facilitate transport into the pore, polypeptide mesh lining allows soluble molecules to pass through freely Molecules imported into the nucleus require a sorting signal called a nuclear localisation signal or NLS -- this is one or two short sequences containing positively charged lysine and arginine amino acids. - The NLS is recognised in the cytosol by a specific preceptor called a nuclear import receptor (NIR) - This process requires energy using the form of GTP - The GTP id used for the receptor to bind the protein in the cytosol - The NIR interacts with the cytosolic fibrils on or near the nuclear pore - This interaction interrupts the interaction between the polypeptide mesh of the pore and opens the passageway to the nucleus - The protein and an NIR complex are translocated through the pore into the nucleus - Once in the nucleus, the protein and NIR separate - NIR returns to cytosol whilst NLS remains attached to the nuclear protein Transport into the mitochondria - Proteins have to cross a double membrane - Unlike the nucleus, proteins have to unfold in order to get into the mitochondria - Proteins have an end terminal signal sequence which allows translocation across both membranes simultaneously at sites where membranes contact each other slightly more closely Transport into the mitochondria sequence - Mitochondrial signal sequence on the protein finds an import receptor in the outer mitochondrial membrane - Import receptor coordinates protein translocators on inner and outer mitochondrial membranes to begin protein translocation - In the matrix, the transported protein binds to chaperone proteins, preventing the protein from translocating back out of the mitochondria into the cytosol - Signal sequence is removed after translocation - After translocation, protein refolds Transport into the ER serves as the entry point to all other organelles of the endomembrane system. Once a protein enters the ER they are either a soluble protein or transmembrane protein -- they tend not to re enter the cytosol. Proteins enter the ER whilst they're being synthesized, and they're attached to ribosomes at the time, giving the ER its rough quality, and it can translocate proteins in: - ER signal sequence in the peptide binds to a special receptor called a signal recognition particle, occurring in the cytosol - This temporarily halts that translation of the messenger RNA, so we've got a ribosome - When the messenger RNA finds the signal recognition particle it slightly slows down the translation of messenger RNA and protein - Signal recognition particle interacts with the signal recognition particle receptor in the ER membrane, releasing the SRP back into the cytosol - The SRP receptor then passes the polypeptide ribosome messenger RNA complex to a protein translocator in the endoplasmic reticulum membrane - This restarts translations so translation starts to initiate again - The ER signal sequence keeps the channel open while the protein is translated and threaded through Transport through vesicles: - Refers to onward transport from the ER - Both soluble and transmembrane proteins from the ER to other compartments of the endomembrane system - Exocytosis: transports proteins in an outward direction called antiretrograde from the ER to the Golgi, to transport vesicles to plasma membrane - Endocytosis: retrograde or inward transport from plasma membrane, vesicles to early endosomes to lysosomes Vesicles: - Distinctive coat proteins on cytosolic surface - Clathrin (coat protein) starts to coat membrane, helping to promote membrane curvature, called coated pitch - Buzz off, producing vesicle Cargo receptors: - Specific cargo receptors in the membrane - Vind both the cargo and a second type of coat protein called adaptins - Adaptins bind cargo receptor and coat protein - Clathrin is further recruited - Small GTP binding protein called dynamin assembles as a ring around the neck of each coat ptoein, causing it to constrict and pinch off, separating vesicle from membrane - All coat proteins are shed as vesicle forms Moodle recap quiz Q: What -- binds cargo proteins during vesicle formation? A: Adaptins Q: The secretory pathway involves the process of - ? A: Exocytosis Q: Protein folding in the ER is assisted by - ? If the peptide doesn't fold properly it is - ? A: Chaperones, degraded Q : Sorting signals are short sequences of - ? that target a peptide to a specific organelle. If a protein doesn't have a sorting signal, it - ? A: Amino acids, remains in the cytosol Q: Disulphide bonds are formed in - ? A: ER Q: Cargo is concentrated at the plasma membrane in the process of - ? A: Receptor mediated endocytosis Q: The mitochondria and nucleus have evolved from bacteria engulfed by eukaryotic cells. True or false? A: False Q: Proteins are transported to the nucleus via vesicles. True or false? A: False, nuclear proteins are targeted via special sorting signals called NLS Q: Proteins enter mitochondria fully folded. True or false? A: False