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Eachof theie is discussedin the text. HoweveLmost cellscontainmore than a hundreddifferent actin-bindin!proteins,and it is rit"rvihaiih*eare importanttypesof actin-associatedproteinsthat are not yet recognized.'pazru6o)alla^ lou oJeleq+suralordpaler)osse-alnqnlor)rur+o sodl+lueuodur sre sraql leLll ^lolrl sr]r-suroloJdpoler)osse-urpeaq] rol se-pue 'surelo.rd6urpurq-elnqnloJ)rurluaJa#rppoJpunqe ueql aJourureluo) slla)+sourla^omoH '1xo+aql ur possn)srpsr osaql+o q)el 'umoqssr ed[1 .rofewq)eo ]o a;duexe ue 'uorposia]el e ur passn)srpaq o1 'suralo.rdrolouJ +o sassel)o,V\lJo+lda)xl 'uola;a1soy{ralnqn}oJ)il,1raq1,rosuralord fuosse>>eroleur eq} +o aulossluaurPlrjalerpauljalur ol s)u.,u r1re1dpue6urlpunq6ur1ur;-sso.oluauelUs a p r s6 u o 1 e6 u r p u r q{ q s a l n q n la z r l r q e } ss a l n q n l o r l r r .
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The first of these is a complex of proteins that includes twoactin-related proteins, or ARPs,each of which is about 45% identical to actin.Analogous to the function of the y-TuRC,the ARP complex (also known as theArp 2/3 complex) nucleates actin filament growth from the minus end, allowingrapid elongation at the plus end (Figure 16-34A and B). The complex can alsoT..-l".,inll]nrpz [arpr []](A)activatingfactormrnusenon u c l e a t e da c t i nf i l a m e n tARPcomplex(B)activeARPcomplex1 0 0n mt.\\!/(D)10 nmFigure 16-34 Nucleationand actin web formation by the ARPcomple;actin.Afthoughthe faceof the moleculeequivalentto the plus end (toP.differenceson the sidesand minus end (bottom)preventtheseactin-reliinto filamentswith actin.(B)A modelfor actinfilamentnucleationby thare held by their accessoryproteinsin an orientationthat preventsthenindicatedby the bluetrianglebinds the complex,Arp2 and Arp3 are broonto this structure,an actinfilament.Actinsubunitscanthen assemblewhen ifilamentsmostefficiently16-10).(C)TheARPcomplexnucleatesroundsof branchingnucleationresultin a treelikeweb offilamentbranchthat growsat a 70.
anglerelativeto the originalfilament.Repeatedwith purifiedARPactin filaments.(D)Top,electronmicrographsof branchedu'.tinfilur"ntt formed by mixing purifiedactin subunitsbeenfitted to thehaveARPcomplextheandactinoistructurescrystalthewherecomplexes.Bottom,reconstructedimageof a branchthe right wherethe ARPcomplexelectrondensity.The mother filament runsfrom top to bottom, and the daughterfilament branchesoff to86, 1998'With permissionbindsto three actin subunitsin the mother filament (D,from R.D.Mullinset al.,Proc.NatlAcad.Sci.IJ.S.A.95:6181-61MacmillanPublishersLtd')frompermissionWith2001.and from N.Volkmannet al.,Science293:2456-2459,from NationalAcademyof Sciences,998Chapter 16:TheCytoskeleton20umrR\1 0 0p mattach to the side of another actin filament while remaining bound to the minusend of the filament that it has nucleated, thereby building individual filamentsinto a treelike web (Figure l6-34C and D).near the plasma membrane in yeast, where it is required to form cortical actinpatches (see Figure 16-6), and in plant cells, where it directs the formation ofactin bundles at the surface that are required for the growth of complex cellshapesin a variety ofdifferent tissues (Figure f6-3b).TheMechanismof NucleationInfruencesLarge-scaleFilamentOrganizationor microtubule and prevent both subunit addition and subunit loss at this end.Figure16-35 Functionof the ARPcomplexin plant cells.(A)Cellsin themaizeleafepidermisform small,actinrichlobesthat lockneighboringcellstogetherlikepiecesof a jigsawpuzzle.(B)The regularpatternof interlockingcellscoversthe leafsurface.(C)Epidermalcellsin a mutantplantlackingthe ARpcomplexdo not form the interlockinglobes.The brick-shapedcellsare normalin sizeand spacing,but form leavesthatappeartoo shinyto the nakedeye.(FromM.J.Frank,H.N.CartwrightandL.G.Smith,Development130:753-762,2003.With permissionfrom the Companyof Biologists.)] + a l )6 u r P u r q - d I Vur aln)elourdfv'a)p!ns auerqurauraql qleauaqAll)eJrpluaureluu!t)e aqt lo uorlezuaullodoql p!epue'auerqureurlPuorlrasureurseldlla) aq] ot pal)auuo)Ill)arlpu!'11ar{ueyt1aql ur sarnlfnrtsole sururroJreln>l1tedo] ]l lull pue ^t!^!l)est!aleln6arsuorbarlaqlg'eraq umoqssraln)elourabre;aqt;o tred ^luO'alqLuasseoturr..tuo;Iunqns Mau Ll)easMollPt! se pua snloaqlle pasooxaslrunqnsur])eoMl aql jo auoo16urpurqsll surelureururaloldurr-u.ro1aq1 sa1e6uo1elr sp pua sn;d6urmor6-{lprderqllmpoler)osseaqtureuarpue(pal)]uaLUPllJullf e Mau e Jo uorlPuuo]aql aleel)nuue) leql xaloulo))llouJpIqe w)o! 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