Ch. 11: Protein Sorting & Transport Online Quiz (Essay) Flashcards
A member of the heat-shock class of proteins, BiP, is necessary for the proper folding of proteins in the ER. In terms of function, what do BiP and the heat-shock proteins (HSP) have in common?
The primary function of heat-shock proteins is to prevent damage to the cell by causing protein ___; in their absence, unfolded proteins would associate into large aggregates that would be difficult to disentangle, once the cell returned to normal conditions. Like BiP, the inducible heat-shock proteins bind unfolded proteins and assist in their proper folding. In fact, BiP and inducible heat-shock proteins are all chaperone proteins. BiP is an Hsp70 protein-family member.
folding
The three contiguous membrane domains of the ER—rough ER, smooth ER, and ER exit sites—are all continuous with one another and, depending on cell type, can be present in greatly differing amounts. For example, in cells that synthesize steroid hormones, the smooth ER is much more abundant. In nonsecretory cells, such as cultured human HeLa cells, there is little ER present. Propose a mechanism by which these domains could be generated and maintained.
The available data does not offer a clear answer to this question. A plausible working hypothesis is that ER exit sites and rough ER arise as ___ within the ER by means of molecular association. In other words, these domains segregate themselves from the smooth ER by self-association.
domains
Describe the roles of clathrin and associated adaptor proteins in the sorting of proteins to lysosomes:
Proteins destined for lysosomes are recognized in the Golgi and acquire a mannose-6-phosphate “tag,” which then binds to the mannose-6-phosphate receptor, a transmembrane protein. As part of the formation of clathrin-coated vesicles at the trans-Golgi network, an adaptor protein is recruited: ___/GTP. The adaptor protein has a binding site for interaction with the cytosolic tail of the mannose-6-phosphate receptor and it also serves as a binding site for assembly of the clathrin coat. The clathrin coat forms a lattice resembling a basket around the adaptor proteins, thus inducing the formation of a vesicle that eventually buds off from the trans-Golgi network. Thus, clathrin plays a role in the physical formation of the vesicle, rather than acting as a selectivity factor. The selectivity factor is the ___ ___.
Arf/GTP, adaptor protein (Arf)
Cargo proteins are transported through the Golgi apparatus to the trans-Golgi network, where they are sorted into various vesicles that are targeted to different destinations. What is the role of vesicles in cargo protein transport through the Golgi apparatus?
Two general models for Golgi function in cargo transport through the organelle are actively being considered: the ___ ___ model and the ___ ___ model. In the stable cisternal model, vesicles are the carriers of cargo proteins between cisternae. In the cisternal maturation model, the cisternae are the actual cargo carriers. Vesicles function to retrieve Golgi components and hence to mature the cisternae as they transport cargo. At present, each model explains some, but not all, characteristics of Golgi transport. Further research is needed.
stable cisternal model, cisternal maturation model
Imagine a transmembrane molecule that lies in the plasma membrane and serves as a receptor for an extracellular signaling molecule. When the ligand-binding domain is inserted into the ER, will it lie within the lumen of the ER or in the cytoplasm?
lumen of the ER
Suppose you are studying a mutant that you know is impaired in secretion, but you do not know the exact nature of the defect. You examine the sugar content of some of the proteins you know to be N-glycosylated and notice that though they normally contain fucose, galactose, and sialic acid residues, these are absent in proteins purified from the mutant. Instead only N-acetylglucosamine and mannose residues are evident. Where do you think the defect in secretion lies?
There is a defect in transport ___ the ___ and ___. In the ER, N-acetylglucosamine, mannose, and glucose residues are added to specific asparagine residues within proteins, but the glucose residues are later removed so that upon reaching the Golgi, they contain only N-acetylglucosamine and mannose residues. Fucose, galactose, and sialic acid residues are added in the Golgi. Thus, an ER-to-Golgi block would explain the lack of these sugar residues in proteins that normally contain them.
between the ER and Golgi
The viral protein hemagglutinin lies in the membrane that surrounds the influenza virus. Hemagglutinin binds to sialic acid residues on membrane proteins of cells lining the respiratory tract, and the virus is subsequently internalized by endocytosis. What is the likely method by which the viral genome, which is now surrounded by two membranes (its own plus the cellular endosomal membrane), gains access to the proteins within the cytosol that it needs in order to replicate?
Once the virus is internalized, the endocytic vesicle makes its way toward the lysosome via the endosomal compartment, and the compartment becomes increasingly acidic. Once the acidity reaches a certain level, hemagglutinin undergoes a conformational change that causes ___ of the two membranes (viral and endosomal), releasing the viral genome into the cytosol. Thus the cell’s attempt to destroy the virus by transporting it to the lysosome actually leads to its destruction.
fusion
In the Golgi apparatus, the enzymes that modify N-linked oligosaccharides are arranged in an assembly-line manner (i.e., in sequential Golgi apparatus subcompartments). In the ER, these enzymes are all present in the same continuous membrane-limited compartment. Propose a reason for this difference.
In the ___, the oligosaccharide precursor to N-linked glycosylation is synthesized and then added to the growing polypeptide cotranslationally. The presence of terminal glucose is used as a “timer” relative to the binding of newly synthesized polypeptide with calnexin during protein folding. The major degradative process is removal of glucose. In the ___, there is extensive trimming of the oligosaccharide sidechain by glycosidases before the sidechain is rebuilt through glycosyltransferase activity. Physical separation of compartments may lead to better separation of reactions.
ER, Golgi
Binding of SRP to the signal sequence arrests the further translation of the nascent polypeptide chain. How does the arrest of translation promote the subsequent translocation of the polypeptide chain through the Sec61 translocation complex?
SRP, in binding to the nascent polypeptide chain and ribosome, arrests polypeptide ___. Only when the SRP has become dissociated from the chain and the nascent chain is being pushed through the translocon does polypeptide ___ resume. This results in an extended conformation of the nascent chain that is being pushed through the translocon. Folding occurs in the ER lumen. If SRP bound but did not arrest translation, there would be the need for chaperones in the cytosol to bind to the nascent chain and keep it in an extended conformation as it continued to elongate. In yeast, where posttranslational insertion occurs, this process is fairly common. In mammalian cells, posttranslational insertion is ___, and the pool of cytosolic chaperones is presumably limited.
elongation, rare
