Ca2+ and calmodulin regulate microtubule-associated protein-actin filament interaction in a flip-flop switch.
MAP2 (microtubule-associated protein 2) and tau factor are calmodulin-binding and actin filament-interacting proteins, respectively. We have examined the effect of Ca2+ and calmodulin on MAP-induced actin gelation by the low-shear falling-ball method, the high-speed centrifugation method, and electron microscopy using negative staining. Each MAP crosslinks actin filaments to increase the apparent viscosities and finally to form gels. Calmodulin inhibited MAP2- and tau factor-induced actin gelation (MAP2- and tau factor-actin interaction) only in the presence of Ca2+, but not in its absence. There were no differences in actin filament crosslinking activity of respective MAPs with or without Ca2+. MAP2 was not coprecipitated with F-actin only in the presence of Ca2+ and calmodulin determined by the high-speed centrifugation method. But MAP2 was found to bind to F-actin under any other conditions examined. In contrast, the tau factor-actin filament interaction could only be detected by the low-shear viscosity, but not by the high-speed centrifugation method. MAP2 and tau factor aggregated to form actin bundles as shown by electron microscopy. MAP2- or tau factor-induced bundle formation of actin filaments was inhibited only in the presence of Ca2+ and calmodulin, but not in the presence or absence of Ca2+. In conclusion, the interaction of MAP2- and tau factor-actin filaments is regulated by Ca2+ and calmodulin in a flip-flop switch.
D000199:Actins / Q000378:metabolism*; D000818:Animals; D001921:Brain / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology*; D002147:Calmodulin / Q000378:metabolism / Q000494:pharmacology*; D002417:Cattle; D004258:DNA Polymerase III; D066298:In Vitro Techniques; D008854:Microscopy, Electron; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D008870:Microtubules / Q000378:metabolism; D008956:Models, Chemical; D011485:Protein Binding; D011487:Protein Conformation; D014157:Transcription Factors / Q000378:metabolism; D014783:Viscosity
D000199:Actins; D002147:Calmodulin; D008869:Microtubule-Associated Proteins; D014157:Transcription Factors; C041273:transcription factor tau; D004258:DNA Polymerase III; D002118:Calcium