B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 51
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But the same typeof structure is now known to be exploited by cells for useful purposes. Eukaryoticcells, for example, store many different peptide and protein hormones that theywill secrete in specialized “secretory granules,” which package a high concentration of their cargo in dense cores with a regular structure (see Figure 13–65). Wenow know that these structured cores consist of amyloid fibrils, which in this casehave a structure that causes them to dissolve to release soluble cargo after beingsecreted by exocytosis to the cell exterior (Figure 3–34A).
Many bacteria use theamyloid structure in a very different way, secreting proteins that form long amyloid fibrils projecting from the cell exterior that help to bind bacterial neighborsinto biofilms (Figure 3–34B). Because these biofilms help bacteria to survive inadverse environments (including in humans treated with antibiotics), new drugsthat specifically disrupt the fibrous networks formed by bacterial amyloids havepromise for treating human infections.Many Proteins Contain Low-complexity Domains that Can Form“Reversible Amyloids”Until recently, those amyloids with useful functions were thought to be eitherconfined to the interior of specialized vesicles or expressed on the exterior of cells,as in Figure 3–34.
However, new experiments reveal that a large set of low complexity domains can form amyloid fibers that have functional roles in both thecell nucleus and the cell cytoplasm. These domains are normally unstructuredand consist of stretches of amino acid sequence that can span hundreds of aminoacids, while containing only a small subset of the 20 different amino acids. In contrast to the disease-associated amyloid in Figure 3–33, these newly discoveredstructures are held together by weaker noncovalent bonds and readily dissociatein response to signals—hence their name reversible amyloids.Many proteins with such domains also contain a different set of domains thatbind to specific other protein or RNA molecules.
Thus, their controlled aggregationabnormal prion formof PrP protein (Prp*)(B) misfolded protein can induce formationof protein aggregatesPrpPrp*heterodimermisfolded protein convertsnormal PrP into abnormalconformationhomodimerthe conversion of morePrP to misfolded formcreates a stable amyloidfibrilprotein aggregate inform of amyloid fibrilsecreted amyloid fibrilreleases soluble peptidehormoneMBoC6 e4.08/3.31PLASMA MEMBRANEamyloid fibril onbacterial surfacefusionsecretorygranuleprocessedpeptidehormoneGolgicisterna(A)peptidoglycanlayerbuddingbacterialmembraneamyloidfibriltemplatesubunit(B)subunitof fibrilFigure 3–34 Two normal functions foramyloid fibrils. (A) In eukaryotic cells,protein cargo can be packed very denselyin secretory vesicles and stored untilsignals cause a release of this cargo byexocytosis. For example, proteins andpeptide hormones of the endocrine system,such as glucagon and calcitonin, areefficiently stored as short amyloid fibrils,which dissociate when they reach the cellexterior.
(B) Bacteria produce amyloid fibrilson their surface by secreting the precursorproteins; these fibrils then create biofilmsthat link together, and help to protect, largenumbers of individual bacteria.PROTEIN FUNCTION133soluble protein withgreen fluorescent tagFUSpre-formed FUSprotein gelSOLUBLE PROTEINREPLACED BY BUFFER(A)dissociation of green protein fromgel is measured by fluorescencemicroscope as a function of timet/2 no dissociationhnRNPA2t/2 = 10.1 minhnRNPA1t/2 = 3.6 min0.51235101520304560time after washing(B)Figure 3–35 Measuring the association between “reversible amyloids.” (A) Experimental setup.
The fiber-forming domainsfrom proteins that contain a low-complexity domain were produced in large quantities by cloning the DNA sequence that encodesthem into an E. coli plasmid so as to allow overproduction of that domain (see p. 483). After these protein domains were purifiedby affinity chromatography, a tiny droplet of concentrated solution of one of the domains (here the FUS low-complexity domain)was deposited onto a microscope dish and allowed to gel. The gel was then soaked in a dilute solution of a fluorescentlylabeled low-complexity domain from the same or a different protein, making the gel fluorescent. After replacing the dilute proteinsolution with buffer, the relative strength of binding of the various domains to each other could then be measured by the decay offluorescence, as indicated. (B) Results.
The low-complexity domain from the FUS protein binds more tightly to itself than it doesto the low-complexity domains from the proteins hnRNPA1 or hnRNPA2. A separate experiment reveals that these three differentRNA binding proteins associate by forming mixed amyloid fibrils. (Adapted from M.Kato et al., Cell 149: 753-767, 2012).in the cell can form a hydrogel that pulls theseother molecules into punctateMBoC6 andn3.318/3.30.6structures called intracellular bodies, or granules. Specific mRNAs can be sequestered in such granules, where they are stored until made available by a controlleddisassembly of the core amyloid structure that holds them together.Consider the FUS protein, an essential nuclear protein with roles in the transcription, processing, and transport of specific mRNA molecules.
Over 80 percent of its C-terminal domain of two hundred amino acids is composed of onlyfour amino acids: glycine, serine, glutamine, and tyrosine. This low complexitydomain is attached to several other domains that bind to RNA molecules. At highenough concentrations in a test tube, it forms a hydrogel that will associate witheither itself or with the low complexity domains from other proteins. As illustratedby the experiment in Figure 3–35, although different low complexity domainsbind to each other, homotypic interactions appear to be of greatest affinity (thus,the FUS low complexity domain binds most tightly to itself ). Further experimentsreveal that that both the homotypic and the heterotypic bindings are mediatedthrough a β-sheet core structure forming amyloid fibrils, and that these structuresbind to other types of repeat sequences in the manner indicated in Figure 3–36.Many of these interactions appear to be controlled by the phosphorylation of serine side chains in the one or both of the interacting partners.
However, a greatdeal remains to be learned concerning these newly discovered structures and thevaried roles that they play in the cell biology of eukaryotic cells.weak cross-betaspineprotein withlow-complexitydomainbinding site forother proteinswith repeatedsequences or forRNA moleculesbound proteinFigure 3–36 One type of complex thatis formed by reversible amyloids.
Thestructure shown is based on the observedinteraction of RNA polymerase with alow-complexity domain of a protein thatregulates DNA transcription. (Adapted fromI. Kwon et al., Cell 155:1049–1060, 2013.)134Chapter 3: ProteinsSummaryA protein molecule’s amino acid sequence determines its three-dimensional conformation. Noncovalent interactions between different parts of the polypeptidechain stabilize its folded structure. The amino acids with hydrophobic side chainstend to cluster in the interior of the molecule, and local hydrogen-bond interactionsbetween neighboring peptide bonds give rise to α helices and β sheets.Regions of amino acid sequence known as domains are the modular units fromwhich many proteins are constructed.
Such domains generally contain 40–350amino acids, often folded into a globular shape. Small proteins typically consistof only a single domain, while large proteins are formed from multiple domainslinked together by various lengths of polypeptide chain, some of which can be relatively disordered. As proteins have evolved, domains have been modified and combined with other domains to construct large numbers of new proteins.Proteins are brought together into larger structures by the same noncovalentforces that determine protein folding.
Proteins with binding sites for their own surface can assemble into dimers, closed rings, spherical shells, or helical polymers. Theamyloid fibril is a long unbranched structure assembled through a repeating aggregate of β sheets. Although some mixtures of proteins and nucleic acids can assemblespontaneously into complex structures in a test tube, not all structures in the cell arecapable of spontaneous reassembly after they have been dissociated into their component parts, because many biological assembly processes involve assembly factorsthat are not present in the final structure.PROTEIN FUNCTIONWe have seen that each type of protein consists of a precise sequence of aminoacids that allows it to fold up into a particular three-dimensional shape, or conformation. But proteins are not rigid lumps of material.
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