ライフサイエンスコーパス: kinetochore

1 d copies to form an "anaphase configuration" kinetochore.

2 g components of the Saccharomyces cerevisiae kinetochore.

3 ome degree of universality in the eukaryotic kinetochore.

4 ibition, enabling MIND to join an assembling kinetochore.

5 ore-centromere relaxed faster than the outer kinetochore.

6 Bub3-Bub1 and Mad1-Mad2 to the budding yeast kinetochore.

7 Mps1-regulated phosphorylation at the outer kinetochore.

8 architecture of the much more complex human kinetochore.

9 hment, yet cannot induce Mad1 loss from that kinetochore.

10 ulate the localization of the Ska complex to kinetochores.

11 suggest that RZZ is dynein's cargo at human kinetochores.

12 c20 fluxing through the same binding site at kinetochores.

13 complex in the fibrous corona of unattached kinetochores.

14 ntribute to the "catch-bond" activity of the kinetochores.

15 attachments by negatively regulating Plk1 at kinetochores.

16 cupancy, and tension at individual mammalian kinetochores.

17 apidly by activating the checkpoint at their kinetochores.

18 s come together linearly to form the base of kinetochores.

19 pNs of poleward-directed force to bioriented kinetochores.

20 ure and study the viscoelastic properties of kinetochores.

21 es to preferentially destabilize misattached kinetochores.

22 enforced by checkpoint signals generated at kinetochores.

23 g growing microtubule plus ends within yeast kinetochores.

24 lexes Bub1-Bub3 and BubR1-Bub3 to unattached kinetochores.

25 The kinetochore, a multiprotein complex that connects centro

26 What happens at kinetochores after Mps1-dependent Bub3-Bub1 recruitment

27 gest that it is generated exclusively by the kinetochores after nuclear envelope breakdown (NEBD).

28 Here, we show that flux of Cdc20 through kinetochores also accelerates mitotic exit by promoting

29 blishment of physical attachment between the kinetochore and dynamic spindle microtubules, which unde

30 r of CENP-C molecules and MTs per Drosophila kinetochore and envisioning kinetochore linkages arrange

31 mitotic transcripts is an important step in kinetochore and spindle assembly and challenge the idea

32 lation to trigger cellular pulling on mutant kinetochores and decouple sisters in vivo, and thereby s

33 tail that is deficient in Ska recruitment to kinetochores and in orienting Ska along protofilaments i

34 mediated dephosphorylation of Mps1 occurs at kinetochores and in the cytosol, and inactivation of bot

35 that participates in microtubule binding at kinetochores and in the mitotic redistribution of the mi

36 The spindle checkpoint senses unattached kinetochores and inhibits the Cdc20-bound anaphase-promo

37 nnections that have been established between kinetochores and microtubles by phosphorylating Dam1.

38 DASH complex bridges the interaction between kinetochores and microtubules, and some in vitro evidenc

39 80 complex, a force-transducing link between kinetochores and microtubules.

40 ance system, monitoring interactions between kinetochores and spindle microtubules and ensuring high-

41 cting protein 1 (KKIP1), associates with the kinetochore, and its depletion causes severe defects in

42 onist approach, avoiding the complexities of kinetochores, and demonstrate that co-recruitment of KNL

43 localized to nuclear envelopes, centrosomes, kinetochores, and midbodies.

44 The checkpoint is activated by unattached kinetochores, and Mps1 kinase phosphorylates KNL1 on con

45 ion in the kinetochore, which we refer to as kinetochore 'architecture', organizes its biochemical ac

46 Kinetochores are complex multiprotein machines that link

47 Kinetochores are dynamic cellular structures that connec

48 Kinetochores are macromolecular assemblies that connect

49 Kinetochores are multiprotein complexes that couple euka

50 will use the relatively simple budding yeast kinetochore as a model, and extrapolate insights gained

51 ' model, in which protofilaments pull on the kinetochore as they curl outward from a disassembling ti

52 rs the attachment of spindle microtubules to kinetochores as a means of detecting errors.

53 By following the fates of individual kinetochores as they congress in live cells, we reveal t

54 dle assembly checkpoint but does require the kinetochore, as depleting kinetochore components prevent

55 Kinetochores assemble at the centromeric region of chrom

56 sent at active centromeres are necessary for kinetochore assembly and cell-cycle progression.

57 cognition of the CENP-A nucleosome to enable kinetochore assembly and centromeric chromatin organizat

58 it integrates phospho-regulatory inputs for kinetochore assembly and disassembly.

59 the centromere, which forms the platform for kinetochore assembly and microtubule attachment.

60 Centromeric chromatin is required for kinetochore assembly during mitosis and accurate chromos

61 However, the molecular mechanisms of kinetochore assembly have not yet been fully elucidated.

62 The model for kinetochore assembly is conserved between humans and yea

63 inner kinetochore is not required for outer kinetochore assembly, we find it is essential to recruit

64 gulatory phosphatase that facilitates proper kinetochore assembly.

65 GT1 degradation and thereby caused defective kinetochore assembly.

66 tates CENP-A(Cse4) nucleosome deposition and kinetochore assembly.

67 orce epigenetic centromere specification and kinetochore assembly.

68 s at centromeres to provide a foundation for kinetochore assembly.

69 hich play key roles in DNA damage repair and kinetochore assembly.

70 e chromosome segregation as a foundation for kinetochore assembly.

71 The spindle and kinetochore-associated (Ska) protein complex is required

72 Methylation of the SKA2 (spindle and kinetochore-associated complex subunit 2) gene has recen

73 The kinetochore-associated kinase Mps1 controls the spindle

74 is SAC defect is caused in part by a loss of kinetochore-associated Mad2 in E6-expressing cells.

75 etochore, is essential for building a proper kinetochore at the centromere in order to direct chromos

76 eosomes play an important role in nucleating kinetochores at centromeres for chromosome segregation.

77 ovides a mechanical flexibility that enables kinetochores at the periphery of the spindle to engage m

78 Kinetochores attach chromosomes to the spindle microtubu

79 mechanism controlling Mps1 inactivation once kinetochores attach to microtubules and the SAC is satis

80 checkpoint (SAC) prevents anaphase until all kinetochores attach to the spindle.

81 by the spindle checkpoint in the presence of kinetochore attachment defects [3, 4].

82 n is sufficient to regulate the stability of kinetochore attachment in vivo.

83 vious studies indicate that the stability of kinetochore attachment is regulated by Aurora B/Ipl1 kin

84 Mechanistically, DISC1 regulates Ndel1's kinetochore attachment, but not its centrosome localizat

85 However, end-on kinetochore attachments have not been observed in Caenor

86 at forces approximately fourfold higher than kinetochore attachments under identical loading conditio

87 tic spindle asymmetry, merotelic microtubule-kinetochore attachments, lagging chromosomes, and aneupl

88 the kinetochore (termed Delta) suggest that kinetochores become stretched by spindle forces and comp

89 s a fibrous corona that assembles on mitotic kinetochores before MT attachment to promote chromosome

90 ing of the molecular components that mediate kinetochore binding [5-7], we do not know how kinetochor

91 bind two Dam1 rings in vitro, and results in kinetochore biorientation and microtubule attachment def

92 nteracted, we compare the roles of two outer-kinetochore bound phosphatases and find that BubR1-assoc

93 es supports functional assembly of the outer kinetochore but is unable to correct errors in kinetocho

94 We propose that Ska is recruited to kinetochores by clusters of Ndc80 proteins and that our

95 1 plays an important role in the assembly of kinetochores by counteracting RNF41-mediated SGT1 degrad

96 other data imply that Stu2 colocalizes with kinetochores by recognizing growing microtubule plus end

97 in Okp1 configures a branch of mitotic inner kinetochores, by tethering Ctf19-Mcm21 and Chl4(CENP-N)-

98 Through this mechanism, the kinetochore can modulate its grip on microtubules over m

99 t minus-end attachment points contributes to kinetochore capture in fission yeast, but the relative c

100 We have developed a biophysical model of kinetochore capture in small fission-yeast nuclei using

101 ity and microtubule rotational diffusion for kinetochore capture, both to the lateral surface of a mi

102 icrotubule rotational diffusion can speed up kinetochore capture, it is unlikely to be the dominant p

103 Interestingly, the inner kinetochore-centromere relaxed faster than the outer kin

104 t targeting of INCENP to microtubules or the kinetochore/centromere promotes the mitotic checkpoint,

105 We find that Shp2 is distributed to the kinetochore, centrosome, spindle midzone, and midbody, a

106 CK1delta kinase Hrr25 is critical for sister kinetochore co-orientation, but its roles are not well u

107 eneration is uncorrelated with the amount of kinetochore-colocalized Stu2.

108 outer kinetochore expands radially and some kinetochores completely lose microtubule attachment, a c

109 The kinetochore complex mediates this interaction.

110 Saccharomyces cerevisiae, expression of the kinetochore complex subunit Ndc80 is downregulated by a

111 hibit a conserved interaction with the Ndc80 kinetochore complex that strengthens its attachment to m

112 ents, which form part of a trypanosome outer kinetochore complex.

113 ation of sumoylated nucleolar RENT and inner kinetochore complexes.

114 se-promoting complex/cyclosome, cohesin, and kinetochore complexes.

115 r kinetochores, representing the final outer kinetochore component recruited prior to anaphase onset.

116 to the Ndc80 complex (Ndc80C), a core outer kinetochore component.

117 lecular explanation for the stoichiometry of kinetochore components and its cell cycle regulation, an

118 t does require the kinetochore, as depleting kinetochore components prevents the error-induced anapha

119 During chromosome segregation, outer kinetochore components track depolymerizing ends of micr

120 removal of erroneous attachments and for the kinetochore composition required to detect tension loss.

121 We speculate that the minimal kinetochore configuration, which exists from G1 through

122 Once the spindle pole-to-kinetochore contact has been made, the homologues of a 4

123 Unattached kinetochores convert the latent open conformer of the ch

124 dings therefore suggest that in this system, kinetochores could be involved in sensing meiotic errors

125 At metaphase, one sister kinetochore couples to depolymerizing microtubules, pull

126 non-phosphorylatable SGT1 mutant rescued the kinetochore defects caused by the loss of PHLPP1.

127 show that the forces at the pole and at the kinetochore depend on the bridging fibre thickness.

128 While the kinetochore-dependent mechanisms that drive mitotic chro

129 PT1), and we show that MYPT1 localization to kinetochores depends on Cyclin A/Cdk1 activity and that

130 he anaphase-promoting complex/cyclosome by a kinetochore-derived "wait anaphase" signal known as the

131 cy of naturally occurring, force-dependent P kinetochore detachment events, while being dispensable f

132 that occur during M-phase exit in metazoans: kinetochore disassembly and nuclear reassembly.

133 ageing, oocytes show increased inter-sister kinetochore distance and premature sister chromatid sepa

134 nst the age-related increase in inter-sister kinetochore distance and PSCS.

135 ic cells so that tensile forces generated at kinetochores do not cause microtubule detachment and del

136 The kinetochore drives faithful chromosome segregation in al

137 mic instability allows search and capture of kinetochores during spindle formation, an important proc

138 ture facilitates quantitative examination of kinetochores during the cell cycle.

139 s are weakened by taxol treatment, the outer kinetochore expands radially and some kinetochores compl

140 cleation from mitotic centrosomes, decreased kinetochore-fiber integrity, increased incidence of chro

141 of inter-microtubule bridges that crosslink kinetochore fibers (K-fibers) are still largely unknown.

142 and the dynamic spindle must robustly anchor kinetochore fibers (k-fibers) to bear this load.

143 However, mitosis continues even when kinetochore fibers are not obviously discernable, and cy

144 stribute forces across them, we propose that kinetochore fibres (k-fibres) exert hundreds of pNs of p

145 Surprisingly, however, we find that kinetochore force generation is uncorrelated with the am

146 amicity of spindle microtubules and also for kinetochore force generation.

147 gene expression program temporally restricts kinetochore function is unknown.

148 The macromolecular kinetochore functions to generate interactions between c

149 after attaching to spindle microtubules, the kinetochore generates the force necessary to move chromo

150 Despite its multitude of functions at kinetochores, how the Ska complex itself is recruited to

151 Immunopurification of KKIP1 from stabilized kinetochores identifies six further components, which fo

152 s including SCF E3 ubiquitin ligases and the kinetochore in eukaryotes.

153 Taken together, our work suggests that each kinetochore in vivo contains two Dam1 rings and that pro

154 g oocyte meiosis I, MEL-28-PP1c disassembles kinetochores in a timely manner to promote elongation of

155 and the adaptor Spindly to recruit dynein to kinetochores in Caenorhabditis elegans embryos and human

156 E localizes to centrosomes, chromosomes, and kinetochores in early mitosis and shuttles to the midzon

157 These findings suggest that kinetochores in organisms such as kinetoplastids are bui

158 roteins, VirD5, localizes to the centromeres/kinetochores in the nucleus of the host cells by its int

159 ier to prevent heterochromatin spreading and kinetochore inactivation at human centromeres.

160 We discovered that in budding yeast, kinetochore inactivation occurs by reducing the abundanc

161 re, we report that CenpH, a component of the kinetochore inner plate, is responsible for G2/M transit

162 How the kinetochore integrates the underlying molecular mechanis

163 istone variant that organizes the centromere/kinetochore interface have been shown to have similar ef

164 by generating a diffusible signal from free kinetochores into the cytoplasm, inhibiting the anaphase

165 The kinetochore is a large, evolutionarily conserved protein

166 The eukaryotic kinetochore is a sophisticated multi-protein machine tha

167 e has a point centromere upon which a single kinetochore is built, which attaches to a single microtu

168 Although a fully assembled inner kinetochore is not required for outer kinetochore assemb

169 We show that Mad1 loss from the kinetochore is switch-like with robust kinetics and that

170 y delaying anaphase onset even when a single kinetochore is unattached to mitotic spindle microtubule

171 bust kinetics and that tension across sister kinetochores is established just before Mad1 loss events

172 , how the Ska complex itself is recruited to kinetochores is unclear.

173 ld complex, as an integral part of the inner kinetochore, is essential for building a proper kinetoch

174 e molecular contacts of this important inner kinetochore joint.

175 chromosomal proteins like DNA polymerases or kinetochore kinases, are demonstrating that ubiquitylati

176 We classify them as kinetochore (KMTs), spindle (SMTs) or astral microtubule

177 The initial kinetochore (KT) encounter with a spindle microtubule (M

178 cancer protein 1 (Hec1) is a subunit of the kinetochore (KT)-associated Ndc80 complex, which ensures

179 Kinetochores (KTs) are large multiprotein complexes that

180 y checkpoint (SAC) proteins by an unattached kinetochore leads to SAC activation.

181 Kinetochores link chromosomes to spindle microtubules an

182 s per Drosophila kinetochore and envisioning kinetochore linkages arranged such that they distribute

183 specific molecular cues or force on specific kinetochore linkages that other attachment geometries ca

184 The kinetochore links chromosomes to dynamic spindle microtu

185 t the Astrin-SKAP complex contains separable kinetochore localization and microtubule binding domains

186 ent in Cdk1 phosphorylation are defective in kinetochore localization but retain microtubule localiza

187 phorylates both Ska1 and Ska3 to inhibit the kinetochore localization of the Ska complex [14].

188 cally during mitosis and is required for the kinetochore localization of the Ska complex.

189 The kinetochore-localized human Ska1 complex binds to microt

190 ol of phosphatase activity associated with a kinetochore-localized nucleoporin contributes to two key

191 c exit by promoting its dephosphorylation by kinetochore-localized protein phosphatase 1, which allow

192 hromosomal loci that promote the assembly of kinetochores, macromolecular complexes that bind spindle

193 By tracking kinetochore movements and using kinetochore markers specific to attachment status, we re

194 We thus propose a model in which kinetochores mature through Ska complex recruitment and

195 nt satisfaction and/or obligatory changes in kinetochore mechanochemistry may occur before dissolutio

196 Kinetochores mediate chromosome congression by either sl

197 is safeguarded by the precise regulation of kinetochore microtubule (k-MT) attachment stability.

198 It requires recruitment of outer kinetochore microtubule binders by centromere proteins C

199 curate segregation rely on the plasticity of kinetochore-microtubule (KT-MT) attachments.

200 le microtubule-binding elements and promotes kinetochore-microtubule attachment [8-11].

201 master spindle checkpoint kinase Mps1 senses kinetochore-microtubule attachment and promotes checkpoi

202 r isolates requires BUB1B to suppress lethal kinetochore-microtubule attachment defects.

203 eveal that both Aurora A and B contribute to kinetochore-microtubule attachment dynamics, and they un

204 trate this remarkable change in the plane of kinetochore-microtubule attachment in human cells are no

205 ho-deficient dam1-3A mutants show stabilized kinetochore-microtubule attachment in vivo.

206 netochore but is unable to correct errors in kinetochore-microtubule attachment in Xenopus egg extrac

207 Stable kinetochore-microtubule attachment is essential for cell

208 a B is regarded as the "master regulator" of kinetochore-microtubule attachment, other mitotic kinase

209 checkpoint highly responsive to Mps1 and to kinetochore-microtubule attachment.

210 raphy to identify the structure of the human kinetochore-microtubule attachment.

211 ate chromosome segregation depends on proper kinetochore-microtubule attachment.

212 KAP complex in ensuring the correct plane of kinetochore-microtubule attachment.

213 Strong kinetochore-microtubule attachments are essential for fa

214 We found that kinetochore-microtubule attachments are established even

215 A key outstanding question is how kinetochore-microtubule attachments are modulated to ens

216 Dephosphorylation is proposed to stabilize kinetochore-microtubule attachments by strengthening ele

217 rophase arrest, chromosome condensation, and kinetochore-microtubule attachments during early prometa

218 vitro microtubule binding, has no effect on kinetochore-microtubule attachments in the Caenorhabditi

219 Precise regulation of kinetochore-microtubule attachments is essential for suc

220 dephosphorylation promotes stabilization of kinetochore-microtubule attachments via the Ska complex

221 omosome biorientation by detaching incorrect kinetochore-microtubule attachments.

222 eration between these two complexes enhances kinetochore-microtubule coupling and is regulated by Aur

223 e demonstrate that Aurora A kinase regulates kinetochore-microtubule dynamics of metaphase chromosome

224 e bipolar attachment after the disruption of kinetochore-microtubule interaction by a microtubule dep

225 However, the molecular basis for robust kinetochore-microtubule interactions remains poorly unde

226 Ndc80 complex to stabilize correctly formed kinetochore-microtubule interactions.

227 Ska complex as a key effector of Mps1 at the kinetochore-microtubule interface, as mutations that mim

228 Distinct kinetochore-microtubule interfaces mediate these behavio

229 ion of NuSAP to the polar force generated at kinetochore microtubules and to the regulation of the po

230 in HeLa and PtK1 cells that a bundle of non-kinetochore microtubules, which we term 'bridging fibre'

231 ll cycle regulation, and highlight how outer kinetochore modules bridge distances of well over 100 nm

232 e mechanism can make a major contribution to kinetochore motility and establish a direct approach for

233 Active forces generated at kinetochores move chromosomes, and the dynamic spindle m

234 By tracking kinetochore movements and using kinetochore markers spec

235 passive interfaces generate friction as the kinetochore moves along microtubules [3, 4].

236 Cancer cell lines with hyperstabilized kinetochore MTs have increased segregation errors and el

237 Centromeres nucleate kinetochores, multi-subunit complexes that capture spind

238 To do so, the kinetochore must hold on to depolymerizing and polymeriz

239 ilitate chromosome movement and segregation, kinetochores must maintain associations with both growin

240 age checkpoint proteins are assembled at the kinetochore, not at damage sites along chromosome arms,

241 KINETOCHORE NULL2 (KNL2) is involved in recognition of c

242 strated by microtubule attachments formed at kinetochores of mitotic chromosomes.

243 spindle until they are bioriented, with the kinetochores of the partners attached to microtubules fr

244 sociated network (CCAN) that forms the inner kinetochore on which outer kinetochore proteins assemble

245 Finally, if the two sister kinetochores on a chromosome are both attached to microt

246 Dynein removes the checkpoint proteins from kinetochores once chromosomes are bioriented.

247 Ska is progressively loaded onto bioriented kinetochore pairs as they congress.

248 inetochore binding [5-7], we do not know how kinetochores physically interact with polymerizing versu

249 We show that phosphorylation of the Ctf19 kinetochore protein by a conserved kinase, DDK, provides

250 CENP-A in the centromeric nucleosome by the kinetochore protein CENP-N.

251 A histone-fold domain interacting with inner kinetochore protein Mif2/CENP-C.

252 t PHLPP1 interacted with the essential outer-kinetochore protein SGT1 and stabilized its protein leve

253 isms by which CENP-A nucleosomes engage with kinetochore proteins are not well understood.

254 t forms the inner kinetochore on which outer kinetochore proteins assemble.

255 ls, the distribution of both inner and outer kinetochore proteins elongates in the absence of microtu

256 amily of proteins distantly related to outer kinetochore proteins Ndc80 and Nuf2.

257 e subunit topology of COMA, bound with inner kinetochore proteins Nkp1 and Nkp2, from the yeast Kluyv

258 tically unclear, and it is not known whether kinetochore proteins other than KNL1 have significant ro

259 Two kinetochore proteins, CENP-C and CENP-N, recognize CENP-

260 oplastids with similarity to canonical outer kinetochore proteins, suggesting some degree of universa

261 dent role in the proper composition of inner kinetochore proteins.

262 segregation requires the proper assembly of kinetochore proteins.

263 Aberrant microtubule-driven kinetochore pushing movements and tripolar mitotic spind

264 teins, we find that Hrr25 phosphorylates the kinetochore receptor for monopolin, Dsn1.

265 A new study reveals the mechanism by which kinetochores recruit the cohesin loader to establish cen

266 sembly checkpoint, but the regulation of its kinetochore recruitment and activity is unclear.

267 he dynactin pointed-end subunit p27 prevents kinetochore recruitment of dynein-dynactin without affec

268 This leads to dynamic kinetochore recruitment of Mad proteins [8, 9], a confor

269 Surprisingly, the kinetochore recruits two Mad1-Mad2 heterotetramers for e

270 e molecular motor dynein concentrates at the kinetochore region of mitotic chromosomes in animals to

271 Together, the data suggest that kinetochore regulation has differential effects on engag

272 ion, whereas in Ska-depleted cells, detached kinetochores remain in a futile reattachment/detachment

273 referentially to properly bi-oriented sister kinetochores, representing the final outer kinetochore c

274 the major microtubule-binding factors of the kinetochore responsible for maintaining chromosome-micro

275 n of its substrate, ARHGEF17, regulates Mps1 kinetochore retention, suggesting an autoregulated, time

276 microtubules and yet does not compromise the kinetochore's ability to grip depolymerizing microtubule

277 The kinetochore scaffold Knl1, when phosphorylated by Mps1,

278 all directions: along the first 3-4 mum from kinetochores, scaling with k-fiber length, and laterally

279 heckpoint arrest can be independent of their kinetochore, spindle pole, and nuclear envelope localiza

280 bust interactions between the macromolecular kinetochore structure and dynamic microtubule polymers.

281 insight into dynamics and plasticity of the kinetochore structure during chromosome segregation in l

282 ion is Aurora B kinase, which phosphorylates kinetochore substrates to promote microtubule turnover.

283 ven when dimerized and is required to target kinetochore substrates.

284 Using a chemically tuned kinetochore-targeting assay, we show that Mps1 destabili

285 istances between various proteins within the kinetochore (termed Delta) suggest that kinetochores bec

286 sting mitotic progression in the presence of kinetochores that are not attached to spindle microtubul

287 osis requires transient removal of the outer kinetochore, the complex that connects microtubules to c

288 The primary microtubule coupler at the kinetochore, the Ndc80 complex, is regulated by Aurora k

289 Kinetochores, the microtubule-interacting machines on ch

290 disassembles nuclei and promotes assembly of kinetochores-the primary microtubule attachment sites on

291 The microtubule attachment status of kinetochores therefore optimizes mitotic duration by con

292 the proteins and their properties that allow kinetochores to associate with dynamic microtubules.

293 results reveal how Mps1 dynamically modifies kinetochores to correct improper attachments and ensure

294 crotubules suggests a mechanism for metazoan kinetochores to couple the depolymerization of microtubu

295 erty that would allow this complex to act at kinetochores to mediate persistent associations with dyn

296 CTD) recruits protein phosphatase 1 (PP1) to kinetochores to promote timely anaphase onset [12].

297 We find that Stu2 colocalizes with kinetochores using its TOG domains, which bind GTP-tubul

298 ds to Ndc80C and recruits the Ska complex to kinetochores where Ska1 can bind both PP1 and microtubul

299 ndependently of motor enzymes, especially at kinetochores where they drive chromosome motility.

300 ow the nanoscale protein organization in the kinetochore, which we refer to as kinetochore 'architect

301 ozoa called kinetoplastids of unconventional kinetochores with no apparent homology to model organism