Biology_Unit_5 (1110837), страница 5
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Mutating other amino acidsin the same region of the proteinimpaired import of the protein intothe nucleus, but did not abolish it.Deleting amino acids 1–126(or any part of that region) or136–708 (or any part of that region): Protein localized to nucleus, meaning that amino acidsin those regions are not importantfor nuclear localization.Deleting amino acids 127–133: Protein localized to cytoplasm, meaningthat this amino acid sequence is necessary for nuclear localization of theviral protein. Other deletions involving parts of this region gave the sameresult.Conclusion: By mutating the viral protein sequence, the researchers identified a seven-amino-acid segment of the protein, amino acids 127–133, that is necessary for localization of the protein to the nucleus.
In follow-up experiments, they added this amino acid sequence to a cellular enzyme protein normallyfound only in the cytoplasm and determined that the modified protein localized to the nucleus. Therefore, the seven-amino-acid sequence is a nuclear localization signal. Continuing research has shown that this sequence is only the first example of similar sequences in other nuclear proteins. Thus, the identification of nuclear localization signals was a key step toward understanding the import of proteins into the nucleus.Sources: D.
Kalderon, W. D. Richardson, A. F. Markham, and A. E. Smith. 1984. Sequence requirements for nuclear location of simian virus 40 large-T antigen. Nature 311:33–38; D. Kalderon,B. L. Roberts, W. D. Richardson, and A. E. Smith, 1984. A short amino acid sequence able to specify nuclear location.
Cell 39:499–509.Eukaryotic Ribosomes Are Either Free in theCytosol or Attached to MembranesLike prokaryotic ribosomes, a eukaryotic ribosome consists of alarge and a small subunit (Figure 5.13). However, the structures ofbacterial, archaeal and eukaryotic ribosomes, although similar,are not identical. In general, eukaryotic ribosomes are larger thaneither bacterial or archaeal ribosomes; they contain four types ofrRNA molecules and more than 80 proteins. Their function isidentical to that of prokaryotic ribosomes: They use the information in mRNA to assemble amino acids into proteins.Some eukaryotic ribosomes are freely suspended in the cytosol; others are attached to membranes.
Proteins made on free98UNIT ONEMOLECULES AND CELLSribosomes in the cytosol may remain in the cytosol, pass throughthe nuclear pores into the nucleus, or become parts of mitochondria, chloroplasts, the cytoskeleton, or other cytoplasmic structures. Proteins that enter the nucleus become part of chromatin,line the nuclear envelope (the lamins), or remain in solution inthe nucleoplasm.Many ribosomes are attached to membranes. Some ribosomes are attached to the nuclear envelope, but most are attachedto a network of membranes in the cytosol called the endoplasmicreticulum (ER) (described in more detail next).
The proteinsmade on ribosomes attached to the ER follow a special path toother organelles within the cell.RibosomeAn Endomembrane System Dividesthe Cytoplasm into Functionaland Structural CompartmentsEukaryotic cells are characterized by an endomembrane system(endo within), a collection of interrelated internal membranoussacs that divide the cell into functional and structural compartments. The endomembrane system has a number of functions,including the synthesis and modification of proteins and theirtransport into membranes and organelles or to the outside of thecell, the synthesis of lipids, and the detoxification of some toxins.The membranes of the system are connected either directly in thephysical sense or indirectly by vesicles, which are small membrane-bound compartments that transfer substances betweenparts of the system.The components of the endomembrane system include thenuclear envelope, endoplasmic reticulum, Golgi complex, lysosomes, vesicles, and plasma membrane.
The plasma membraneand the nuclear envelope are discussed earlier in this chapter.The functions of the other organelles are described in the following sections.ENDOPLASMIC RETICULUM The endoplasmicreticulum (ER) is an extensive interconnectednetwork (reticulum little net) of membranous channels and vesicles called cisternae (singular, cisterna).
Each cisterna isRough ERformed by a single membrane thatsurrounds an enclosed space calledthe ER lumen (Figure 5.14). The ERoccurs in two forms: rough ERand smooth ER, each with speA. Rough ERcialized structure and function.The rough ER (see Figure5.14A) gets its name from themany ribosomes that stud its outersurface. The proteins made on ribosomes attached to the ER enterthe ER lumen, where they fold intotheir final form. Chemical modifications of these proteins, such asRibosomesaddition of carbohydrate groupsRibosomeFIGURE 5.13Large subunitA ribosome. The diagramshows the structures of thetwo ribosomal subunits ofmammalian ribosomes andhow they come together toform the whole ribosome.Small subunitRough ERSmooth ERB.Smooth ERSmooth ERRough ER lumenSmooth ER lumenCisternaeCisternaeDon W.
Fawcett/Visuals UnlimitedDon W. Fawcett/Visuals UnlimitedFIGURE 5.14The endoplasmic reticulum. (A) RoughER, showing the ribosomes that studthe membrane surfaces facing thecytoplasm. Proteins synthesized onthese ribosomes enter the lumen ofthe rough ER where they are modifiedchemically and then begin their pathto their final destinations in the cell.(B) Smooth ER membranes.
Amongtheir functions are the synthesis oflipids for cell membranes, and enzymatic conversion of certain toxicmolecules to safer molecules.Vesicle budding from rough ERRibosomeSmooth ER lumenCHAPTER 50.5 μmTHE CELL: AN OVERVIEW99GOLGI COMPLEX Camillo Golgi, a late-nineteenth-century Ital-ian neuroscientist and Nobel laureate, discovered the Golgi complex. The Golgi complex consists of a stack of flattened, membranous sacs (without attached ribosomes) known as cisternae(Figure 5.15). In most cells, the complex looks like a stack of cuppedpancakes, and like pancakes, they are separate sacs, not interconnected as the ER cisternae are. Typically there are between threeand eight cisternae, but some organisms have Golgi complexeswith several tens of cisternae.
The number and size of Golgi complexes can vary with cell type and the metabolic activity of thecell. Some cells have a single complex, whereas cells highly activein secreting proteins from the cell can have hundreds of complexes. Golgi complexes are usually located near concentrationsof rough ER membranes, between the ER and the plasmamembrane.The Golgi complex receives proteins that were made in theER and transported to the complex in vesicles. When the vesiclescontact the cis face of the complex (which faces the nucleus), theyfuse with the Golgi membrane and release their contents directlyinto the cisternal (see Figure 5.15). Within the Golgi complex,the proteins are chemically modified, for example, by removingsegments of the amino acid chain, adding small functionalgroups, or adding lipid or carbohydrate units.
The modified pro10 0UNIT ONEMOLECULES AND CELLSteins are transported within the Golgi to the trans face of thecomplex (which faces the plasma membrane), where they aresorted into vesicles that bud off from the margins of the Golgi(see Figure 5.15). The content of a vesicle is kept separate from thecytosol by the vesicle membrane. Th ree quite different modelshave been proposed for how proteins move through the Golgicomplex. The mechanism is a subject of active current research.The Golgi complex regulates the movement of several typesof proteins. Some are secreted from the cell, others become embedded in the plasma membrane as integral membrane proteins,and yet others are placed in lysosomes.
The modifications of theproteins within the Golgi complex include adding “zip codes” toGolgi complexRough ERSmooth ERGolgi complexcis face—vesiclesfrom ER fuse withthis sideVesicle fromER, about tofuse with theGolgi membraneCisternaeVesicles buddedfrom Golgi containingfinished productInternalspacetrans face—vesicles leaveGolgi from thisside for othercell locationsBiophoto Associates/Photo Researchers, Inc.to produce glycoproteins, occur in the lumen. The proteins are thendelivered to other regions of the cell within small vesicles that pinchoff from the ER, travel through the cytosol, and join with the organelle that performs the next steps in their modification and distribution.
For most of the proteins made on the rough ER, the nextdestination is the Golgi complex, which packages and sorts themfor delivery to their final destinations.The outer membrane of the nuclear envelope is closely related in structure and function to the rough ER, to which it isconnected. This membrane is also a “rough” membrane, studdedwith ribosomes attached to the surface facing the cytoplasm. Theproteins made on these ribosomes enter the space between thetwo nuclear envelope membranes. From there, the proteins canmove into the ER and on to other cellular locations.The smooth ER (see Figure 5.14B) is so called because itsmembranes have no ribosomes attached to their surfaces. Thesmooth ER has various functions in the cytoplasm, includingsynthesis of lipids that become part of cell membranes.
In somecells, such as those of the liver, smooth ER membranes containenzymes that convert drugs, poisons, and toxic by-products ofcellular metabolism into substances that can be tolerated or moreeasily removed from the body.The rough and smooth ER membranes are often connected,making the entire ER system a continuous network of interconnected channels in the cytoplasm. The relative proportions ofrough and smooth ER reflect cellular activities in protein andlipid synthesis.
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