How are most eukaryotic plasma membrane proteins synthesized?
Most eukaryotic plasma membrane proteins are synthesized on a. free ribosomes and inserted after translation into the plasma membrane. b. rough ER ribosomes and carried to the plasma membrane by vesicles that pinch off from the Golgi apparatus. c. rough ER ribosomes and carried to the plasma membrane by vesicles that pinch off
How are membrane proteins made?
Membrane proteins are synthesized on the ribosomal machinery of cells and then inserted into membranes. In eukaryotic cells, proteins are either first inserted co-translationally into the membrane of the endoplasmic reticulum, or post-translationally into membranes of mitochondria, the nucleus, or peroxisomes.
How are secretory and new plasma membrane proteins synthesized?
Secretory proteins and new plasma membrane proteins are synthesized by ribosomes that attach to the inner surface of the plasma membrane. As in eukaryotic cells, ribosome attachment follows synthesis of a signal peptide encoded in the protein’s mRNA. The protein is then dispatched through the cell membrane and into the extracellular space.
Where are proteins synthesized in the Golgi apparatus?
Rough ER, Golgi apparatus, secretory vesicles, plasma membrane (Proteins are synthesized in the rough ER, modified in the Golgi apparatus, and carried in secretory vesicles to the plasma membrane, where they are secreted)
Where are membrane proteins located?
Other membraneproteins are located entirely in the cytosoland are associated with the cytosolic monolayer of the lipid bilayereither by an amphipathicα helixexposed on the surface of the protein(example 4 in Figure 10-17) or by one or more covalently attached lipid chains, which can be fatty acidchains or prenyl groups(example 5 in Figure 10-17and Figure 10-18). Yet other membrane proteins are entirely exposed at the external cell surface, being attached to the lipid bilayer only by a covalent linkage(via a specific oligosaccharide) to phosphatidylinositolin the outer lipid monolayer of the plasma membrane(example 6 in Figure 10-17).
What are membrane proteins?
It is the proteins, therefore, that give each type of membrane in the cell its characteristic functional properties .
How to determine the sidedness of a protein?
One method is the use of a covalent labeling reagent (such as a radioactive or fluorescent marker) that is water-soluble and therefore cannot penetrate the lipid bilayer; such a marker attaches covalently only to the portion of the protein on the exposed side of the membrane. The membranes are then solubilized with detergentand the proteins are separated by SDS polyacrylamide-gel electrophoresis. The labeled proteins can be detected either by their radioactivity (by autoradiographyof the gel) or by their fluorescence (by exposing the gel to ultraviolet light). By using such vectorial labeling,it is possible to determine how a particular protein, detected as a band on a gel, is oriented in the membrane: for example, if it is labeled from both the external side (when intact cells or sealed ghosts are labeled) and the internal (cytosolic) side (when sealed inside-out vesicles are labeled), then it must be a transmembrane protein. An alternative approach is to expose either the external or internal surface to proteolytic enzymes, which are membrane-impermeant: if a protein is partially digested from both surfaces, it must be a transmembrane protein. In addition, labeled antibodies that bind only to one part of a protein can be used to determine whether that part of a transmembrane protein is exposed on one side of the membrane or the other.
What are the functions of proteins in the cell membrane?
Although the basicstructure of biological membranes is provided by the lipid bilayer, membraneproteins perform most of the specific functions of membranes. It is the proteins, therefore, that give each type of membrane in the cell its characteristic functional properties. Accordingly, the amounts and types of proteins in a membrane are highly variable. In the myelin membrane, which serves mainly as electrical insulation for nerve cellaxons, less than 25% of the membrane mass is protein. By contrast, in the membranes involved in ATP production (such as the internal membranes of mitochondria and chloroplasts), approximately 75% is protein. A typical plasma membraneis somewhere in between, with protein accounting for about 50% of its mass.
What are the most useful molecules for membrane biochemists?
The most useful of these for the membrane biochemist are detergents, which are small amphipathicmolecules that tend to form micelles in water (Figure 10-23). When mixed with membranes, the hydrophobic ends of detergents bind to the hydrophobic regions of the membrane proteins, thereby displacing the lipid molecules.
Why are there more lipids than protein molecules?
Because lipidmolecules are small compared with proteinmolecules, there are always many more lipid molecules than protein molecules in membranes—about 50 lipid molecules for each protein moleculein a membranethat is 50% protein by mass. Like membrane lipids, membrane proteins often have oligosaccharidechains attached to them that face the cell exterior. Thus, the surface that the cell presents to the exterior is rich in carbohydrate, which forms a cell coat,as we discuss later.
Why are red blood cells only found in plasma membrane?
Red blood cells (also called erythrocytes) are available in large numbers (from blood banks, for example) relatively uncontaminated by other cell types. Since they have no nucleusor internal organelles, the plasma membrane is their only membrane, and it can be isolated without contamination by internal membranes (thus avoiding a serious problem encountered in plasma membrane preparations from other eukaryotic cell types, in which the plasma membrane typically constitutes less than 5% of the cell's membrane).
Where are proteins inserted in the cell membrane?
Membrane proteins are synthesized on the ribosomal machinery of cells and then inserted into membranes. In eukaryotic cells, proteins are either first inserted co-translationally into the membrane of the endoplasmic reticulum, or post-translationally into membranes of mitochondria, the nucleus, or peroxisomes. From the endoplasmic reticulum, membrane proteins may travel in vesicles to the Golgi complex, the lysosome, or the cell surface, and on the way may be subjected to post-translational modification. In bacteria, proteins assemble either into the cytoplasmic or outer membrane. The final location and topography depends on targeting sequences, hydrophobic domains, or covalently-linked lipid.
Which membrane is the signal sequence binding protein integrated into?
Identification of signal sequence binding proteins integrated into the rough endoplasmic reticulum membrane
What is required for budding from Golgi membranes?
Budding from Golgi membranes requires the coatomer complex of non-clathrin coat proteins
What protein contains mitochondrial import signal?
Precise determination of the mitochondrial import signal contained in a 70 kDa protein of yeast mitochondrial outer membrane
Which sequence mediate delayed translocation and retention of the protein in the ER?
Sequences in rotavirus glycoprotein VP7 that mediate delayed translocation and retention of the protein in the ER
Which protein is sufficient to direct cytosolic dihydrofolate reductase into the mitochondrial?
The cleavable prepiece of an imported mitochondrial protein is sufficient to direct cytosolic dihydrofolate reductase into the mitochondrial matrix
Which structure determines the topological orientation of cytochrome P-450 in microsomal membranes?
The amino-terminal structures that determine topological orientation of cytochrome P-450 in microsomal membranes
What is the name of the protein that synthesizes on one surface and flipped to the other surface?
d. synthesized on one surface and flipped to the other surface by proteins called flippases.
When does secretory protein end up in the cytosol?
b. The secretory protein ends up in the cytosol when a short sequence is deleted by genetic engineering.
Where is c secreted?
c. be secreted through channels in the plasma membrane.
Where are proteins secreted from the cell?
Proteins that will be secreted from the cell are likely to be found in closed spaces bounded by membranes of the endomembrane system
What was the final destination of the proteins?
The final destination of the proteins was the lysosome
What cell secretes a large amount of digestive enzymes?
lysosome. (prefix lyso- means decomposition) Pancreatic cell, which secrete a large amount of digestive enzymes, are labeled with radioactive leucine and then chased for several hours with non-radioactive leucine.
Where does digestion occur?
Digestion of the food particle occurs in a vesicle enclosed by a membrane that separates the digestion from the cytoplasm
