Initial diameter of the polar body contractile ring is minimized by the centralspindlin complex

摘要

Polarbodyformationisanessentialstepinforminghaploideggsfromdiploidoocytes.Thisprocessinvolvescompletionofahighlyasymmetriccytokinesisthatresultsinalargeeggandtwosmallpolarbodies.Unlikemitoticcontractilerings,polarbodycontractileringsassembleoveronespindlepolesothatthespindlemustmovethroughthecontractileringbeforecytokinesis.Duringtime-lapseimagingofemC.elegans/emmeiosis,thecontractileringmoveddownwardalongthelengthofthespindleandcompletedscissionatthemidpointofthespindle,evenwhenspindlelengthorrateofringmovementwasincreased.Patchesofmyosinheavychainanddynamicfurrowingoftheplasmamembraneovertheentireembryosuggestedthatglobalcorticalcontractionforcesthemeioticspindleandoverlyingmembraneoutthroughthecontractileringcenter.Consistentwiththismodel,depletionofmyosinphosphataseincreasedthevelocityofringmovementalongthelengthofthespindle.Globaldynamicfurrowing,whichwasrestrictedtoanaphaseIandII,wasdependentonmyosinII,theanaphasepromotingcomplexandseparase,butdidnotrequirecorticalcontactbythespindle.LargecorticalpatchesofmyosinduringmetaphaseIandIIindicatedthatmyosinwasalreadyintheactiveformbeforeactivationofseparase.Toidentifythesignalatthemidpointoftheanaphasespindlethatinducesscission,wedepletedtwoproteinsthatmarktheexactmidpointofthespindleduringlateanaphase,CYK-4andZEN-4.Depletionofeitherproteinresultedintheunexpectedphenotypeofinitialingressionofapolarbodyringwithtwicethediameterofwildtype.Thisphenotyperevealedanovelmechanismforminimizingpolarbodysize.Proteinsatthespindlemidpointarerequiredforinitialringingressiontooccurclosetothemembrane-proximalspindlepole./p/div