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Others control microtubule positioningby helping to capture and stabilize the growing microtubule end at the locationof specific target proteins in the cell cortex. EBl, a +TIP present in both yeastsand humans, for example, is essential for yeast mitotic spindle positioning,directing the growing plus ends of yeast spindle microtubules to a specific docking region in the yeast bud and then helping to anchor them there.in CellsStructuresinto Higher-OrderAreOrganizedFilamentsSo far, we have described how cells use accessoryproteins to regulate the location and dynamic behavior of cltoskeletal filaments. These proteins can nucleate filament assembly,bind to the ends or sides of the filaments, or bind to thefree subunits of filaments.
But in order for the cytoskeletal filaments to form auseful intracellular scaffold that gives the cell mechanical integrity and determines its shape,the individual filaments must be organized and attached to oneanother in larger-scale structures. The centrosome is one example of such acytoskeletal organizer; in addition to nucleating the growth of microtubules, itholds them together in a defined geometry, with all of the minus ends buried inthe centrosome and the plus ends pointing outward. In this way, the centrosomecreatesthe astral array of microtubules that is able to find the center of each cell(seeFigure 16-32).Another mechanism that cells use to organize filaments into large structuresis filament cross-linking.
As described earlier, some MAPs can bundle microtubules together: they have two domains-one that binds along the microtubuleside (and thereby stabilizes the filament) and another that projects outward tocontact other MAP-coated microtubules. In the actin cytoskeleton, the stabilizing and cross-linking functions are separated. Tropomyosin binds along thesides of actin filaments, but it does not have an outward projecting domain. Aswe shall see shortly, filament cross-linking is instead mediated by a secondgroup of actin-binding proteins that have only this function. Intermediate filaments are different yet again; they are organized both by a lateral self-association of the filaments themselves and by the cross-linking activity of accessoryproteins, as we describe next.and BundledIntoFilamentsAreCross-LinkedlntermediateStrongArraysEach individual intermediate filament forms as a long bundle of tetrameric subunits (see Figure 16-19).
Many intermediate filaments further bundle themselves by self-association; for example, the neurofilament proteins NF-M andNF-H (seeTable 16-I, p. 985) contain a C-terminal domain that extends outwardfrom the surface of the assembled intermediate filament and binds to a neighboring filament. Thus groups of neurofilaments form robust parallel arrays thatare held together by multiple lateral contacts, giving strength and stability to thelong cell processesofneurons (seeFigure 16-22).Other tlpes of intermediate filament bundles are held together by accessoryproteins, such as filaggrin, which bundles keratin filaments in differentiatingcells of the epidermis to give the outermost layers of the skin their special toughness.Plectinis a particularly interesting cross-linking protein.
Besidesbundlingintermediate filaments, it also links the intermediate filaments to microtubules,actin filament bundles, and filaments of the motor protein myosin II (discussedbelow), as well as helping to attach intermediate filament bundles to adhesivestructures at the plasma membrane (Figure 16-46).Mutations in the gene for plectin cause a devastating human disease thatcombines epidermolysis bullosa (caused by disruption of skin keratin filaments), muscular dystrophy (caused by disruption of desmin filaments), andneurodegeneration (caused by disruption of neurofilaments).
Mice lacking a10051006Chapter16:TheCytoskeletonu . 5u mFigure16-46 Plectincross-linkingofdiversecytoskeletalelements.Plectin(green)is seenhere makingcross-linksfrom intermediatefilaments(blue)tomicrotubules(red).ln this electronmicrograph,the dots (yellow)are goldparticleslinkedto anti-plectinantibodies.Theentireactinfilamentnetworkwasremovedto revealtheseproteins.(FromT.M.Svitkinaand G.G.Borisy,J.
CellBiol.135:991-1007,1996.With permissionfrom The RockefellerUniversityPress.)functional plectin gene die within a few days of birth, with blistered skin andabnormal skeletal and heart muscles.Thus, although plectin may not be necessary for the initial formation and assembly of intermediate filaments, its crosslinking action is required to provide cells with the strength they need to withstand the mechanical stressesinherent to vertebrate life.cross-linkingProteinswith DistinctpropertiesorganizeDifferentAssembliesof ActinFilamentsActin filaments in animal cells are organized into two types of arrays: bundlesand weblike (gel-like) networks (Figure lHz).
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