Molecular Biology Wk 7 Flashcards
Proteins Are Transported into Organelles by Three Mechanisms
Protein targeting or protein sorting is the biological mechanism by which proteins are transported to their appropriate destinations in the cell or outside it. The synthesis of virtually all proteins in the cell begins on ribosomes in the cytosol. The fate of any protein molecule synthesized in the cytosol depends on its amino acid sequence, which can contain a sorting signal that directs the protein to the organelle in which it is required.
- Nuclear pores
- Protein translocators
3.vesicular transport
Transport Vesicles Carry Soluble Proteins and Membrane Between Compartments
Vesicular transport is highly organized. Transport vesicles bud from one membrane and fuse with another, carrying membrane components and soluble proteins between compartments of the endomembrane system and the plasma membrane.
In the outward secretory pathway (red arrows), protein molecules are transported from the ER, through the Golgi apparatus, to the plasma membrane or (via early and late endosomes) to lysosomes. In the inward endocytic pathway (green arrows), extracellular molecules are ingested (endocytosed) in vesicles derived from the plasma membrane and are delivered to early endosomes and, usually, on to lysosomes via late endosomes.
Vesicle Budding Is Driven by the Assembly of a Protein Coat
Vesicles that bud from membranes usually have a distinctive protein coat on their cytosolic surface and are therefore called coated vesicles. After budding from its parent organelle, the vesicle sheds its coat, allowing its membrane to interact directly with the membrane to which it will fuse. Cells produce several kinds of coated vesicles, each with a distinctive protein coat. The coat serves at least two functions: 1. it helps shape the membrane into a bud and 2. captures molecules for onward transport
Clathrin triskelions
composed of 3 heavy chains and 3 light chains, are the basic subunits of the clathrin coat. Derived from the Greek word “Triskeles” means “three legs“. A small GTP-binding protein called dynamin assembles as a ring around the neck of each deeply invaginated coated pit. Together with other proteins recruited to the neck of the vesicle, the dynamin causes the ring to constrict, thereby pinching off the vesicle from its parent membrane.
Clathrin-coated vesicles transport selected cargo molecules
Cargo receptors, with their bound cargo molecules, are captured by adaptins, which also bind clathrin molecules to the cytosolic surface of the budding vesicle.
Dynamin proteins assemble around the neck of budding vesicles and with the help of other proteins recruited to the neck (not shown), pinch off the vesicle. There are different types of adaptins: the adaptins that bind cargo receptors in the plasma membrane, for example, are not the same as those that bind cargo receptors in the Golgi apparatus, reflecting the differences in the cargo molecules to be transported from each of these sources.
Adaptin proteins are specific to destination
Clathrin coated - clathrrin and adaptin 1 so origin is Golgi to lysosomes
Clathrin- coated- clathrin and adapting 2 so origin is from P.M to endosomes
COP coated- COP proteins so origin is from ER, cisternae of Golgi and Golgi to er to Golgi, Golgi to Golgi and then Golgi to ER
Vesicle Docking Depends on Tethers and SNAREs
Each type of transport vesicle carries a unique combination of Rab proteins, which serve as molecular markers for each membrane type.
A filamentous tethering protein on a membrane binds to a Rab protein on the surface of a vesicle. This interaction allows the vesicle to dock on its particular target membrane. A v-SNARE on the vesicle then binds to a complementary t-SNARE on the target membrane. Whereas Rab and tethering proteins provide the initial recognition between a vesicle and its target membrane, complementary SNARE proteins ensure that transport vesicles dock at their appropriate target membranes. These SNARE proteins also catalyze the final fusion of the two membranes
Following vesicle docking, SNARE proteins can catalyze the fusion of the vesicle and target membranes
After vesicle docking, the fusion of a vesicle with its target membrane sometimes requires a special stimulatory signal. Whereas docking requires only that the two membranes come close enough for the SNAREs protruding from the two lipid bilayers to interact, fusion requires a much closer approach: the two bilayers must come within 1.5 nm of each other so that their lipids can intermix. For this close approach, water must be displaced from the hydrophilic surfaces of the membranes—a process that is energetically highly unfavorable and thus prevents membranes from fusing randomly.
All membrane fusions in cells must therefore be catalyzed by specialized proteins that assemble to form a fusion complex, which provides the means to cross this energy barrier. The SNARE proteins themselves catalyze the fusion process: once fusion is triggered, the v-SNAREs and t-SNAREs wrap around each other. After fusion, the SNAREs are pried apart so that they can be used again.
Modification of proteins
Many of the proteins that enter the ER lumen or ER membrane are converted to glycoproteins.
The oligosaccharide is originally attached to a specialized lipid, called dolichol, in the ER membrane.
Many proteins are glycosylated on asparagines in the ER. When an appropriate asparagine enters the ER lumen, it is glycosylated by addition of a branched oligosaccharide side chain. Each oligosaccharide chain is transferred as an intact unit to the asparagine from a lipid called dolichol, catalyzed by the enzyme oligosaccharyl transferase.
Exit from the ER Is Controlled to Ensure Protein Quality
Proteins that fold incorrectly are bound to chaperone proteins which ensure and hold proteins in the ER until proper folding occurs if not they are degraded
Antibody molecules, for example, are composed of four polypeptide chains that assemble into the complete antibody molecule in the ER. Partially assembled antibodies are retained in the ER until all four polypeptide chains have assembled; any antibody molecule that fails to assemble properly is degraded. In this way, the ER controls the quality of the proteins that it exports to the Golgi apparatus.
Cystic fibrosis
Cystic
fibrosis Autosomal recessive
disease CFTR gene
Sometimes this quality control mechanism can be detrimental to the organism. For example, the predominant mutation that causes the common genetic disease cystic fibrosis, which leads to severe lung damage, produces a plasma-membrane transport protein that is slightly misfolded; even though the mutant protein could function normally as a chloride channel if it reached the plasma membrane, it is retained in the ER, with dire consequences.
Cystic fibrosis (CF) is a genetic disorder that affects mostly the lungs, but also the pancreas, liver, kidneys, and intestine. Long-term issues include difficulty breathing and coughing up mucus as a result of frequent lung infections. Other signs and symptoms may include sinus infections, poor growth, fatty stool, clubbing of the fingers and toes, and infertility in some males.
Cystic fibrosis
The CFTR gene provides instructions for making a channel that transports negatively charged particles called chloride ions into and out of cells. The most common mutation is a deletion of three nucleotides that results in a loss of the amino acid phenylalanine (F). Cytogenetic Location: 7q31.2 which is the long (q) arm of chromosome 7 at position 31.2.
How is cystic fibrosis diagnosed?
The diagnosis of CF requires clinical symptoms consistent with CF in at least one organ system and evidence of CFTR dysfunction usually based on an abnormal sweat chloride test or the presence of mutations in the CFTR gene.
Clinical symptoms aren’t required for infants identified through newborn screening.
The sweat chloride test is the most commonly used test for diagnosing cystic fibrosis. It checks for increased levels of salt in the sweat. The test is performed by using a chemical that makes the skin sweat when triggered by a weak electric current.
Sweat is collected on a pad or paper and then analyzed. A diagnosis of cystic fibrosis is made if the sweat is saltier than normal
How is cystic fibrosis treated?
Medications:
➢Antibiotics
➢Mucus-thinning medications
➢Nonsteroidal anti-inflammatory drugs (NSAIDs)
➢Bronchodilators relax the muscles around the tubes that carry air to the lungs, which helps increase airflow. Patient can take this medication through an inhaler or a nebulizer.
➢Cystic fibrosis transmembrane conductance regulator (CFTR) modulators - are a class of drugs that act by improving the function of the defective CFTR gene. These drugs represent an important advance in management of cystic fibrosis because they target the function of the mutant CFTR gene rather than its clinical consequences.
The Size of the ER Is Controlled by the Demand for Protein
Accumulation of misfolded proteins in the ER lumen triggers an unfolded protein response (UPR). This program prompts the cell to produce more ER, including more chaperones and other proteins concerned with quality control . The misfolded proteins are recognized by several types of transmembrane sensor proteins in the ER membrane, each of which activates a different part of the UPR.
Some sensors stimulate the production of transcription regulators that activate genes encoding chaperones or other proteins of the ER quality control system. Another sensor also inhibits protein synthesis, reducing the flow of proteins through the ER.
