The appearance of a functional contractile apparatus in developing muscle☆

[摘要] Thecontractileapparatusinmuscleiscomposedofactin,myosin,tropomyosin,troponin,andotherminorproteins.Animportantaspectinstudyingtheappearanceofafunctionalcontractileapparatus,whichinvolvesassemblyofamulticomponentstructure,istodeterminewhatproteinsareratelimiting.TheobservationthattheMg2+Ca2+ATPaseofnaturalactomyosindoesnotchangeinspecificactivityfrom9daysofembryogenesistotheadultsuggeststhatactinandmyosinarenotratelimiting.Thissupportsresultsreportedelsewhereintheliterature.Evidenceispresentedwhichsuggeststhattroponinisratelimitingandbecomesincorporatedintotheactomyosincomplexlateindevelopment.ThepresenceofaCa2+sensitiveATPase,aninvitroequivalentofrelaxation,isusedasanassayforafunctionalcontractileapparatus.ThereisalowlevelofCa2+sensitivityin9-and11-dayactomyosin.Thisthenincreasesmarkedlyandbyday16hasreachedtheadultlevel.Ca2+sensitivityofdesensitizedactomyosincanberestoredbyadditionofrelaxingfactorproteinsystem(RPS=troponinandtropomyosin).WhenRPSisaddedto19-dayandadultactomyosins,theCa2+sensitiveATPaseisunchanged.Whenaddedto14-dayactomyosin,Ca2+sensitivityisrestoredtotheadultlevel.Thisindicatesthat14-dayactomyosinisdeficientintroponinortropomyosinandtroponin.AdditionofRPSto9-and11-dayactomyosinpartiallyrestoresCa2+sensitivitybutnottotheadultlevel,suggestingthatanotherfactorisalsoinvolved.Troponinhastheabilitytobindcalcium.Studyof45Cabindingbynaturalactomyosinshasshownthat14-daybindslessthanadultandmorethan9-and11-daymaterial.Thepointsfor16-dayand19-dayareinbetween14-dayandadultactomyosin.Theseresultsshowthatembryonicactomyosinfromday14andearlierhasreducedCa2+sensitivityandCabindingability.CalciumsensitivitycanberestoredbyadditionofRPS,suggestingfurtherthattroponin(ortropomyosin-troponincomplex)isdeficientintheembryonicnaturalactomyosin.

[学科分类] 生物科学（综合）