title: Calcium sensitivity of sea urchin tubulin in in vitro assembly and the effects of calcium-dependent regulator (CDR) proteins isolated from sea urchin eggs and porcine brains.
abstract: Tubulin was purified from unfertilized eggs or embryos of the sea urchin. In vitro assembly of sea urchin tubulin into microtubules (MTs) was highly sensitive to Ca2+ ions. At low ionic strength, the self-assembly was inhibited by 5 x 10(-7) M free Ca2+, and MT elongation which was initiated by mixing sea urchin tubulin with Tetrahymena cilia outer fiber fragments was inhibited by 10(-6)--10(-5) M free Ca2+. Increase in the ionic strength of the assembly medium lowered the Ca2+ sensitivity, in contrast to brain MT assembly. Brain microtubule-associated proteins (MAPs), which stimulated the sea urchin MT assembly, lowered the Ca2+ sensitivity. Calcium-dependent regulator (CDR) protein was purified from unfertilized eggs of the sea urchin by the pruification procedures used for brain CDR. The sea urchin CDR associated with brain tubulin in a Ca2+-dependent manner indistinguishable from that of brain CDR when assayed by the ammonium sulfate fractionation method. Moreover, both CDRs had Ca2+-dependent inhibitory effects on brain MT assembly. However, neither procine brain CDR nor sea urchin CDR showed a Ca2+-dependent inhibitory effect on sea urchin MT assembly. Sea urchin tubulin assembly initiated by brain MAPs was not inhibited by these CDRs with Ca2+, either. These results suggest that CDRs are not species-specific, but that tubulins respond in a highly specific manner.
mesh_terms: D000818:Animals; D001923:Brain Chemistry; D002118:Calcium; D002135:Calcium-Binding Proteins; D002147:Calmodulin; D005260:Female; D007700:Kinetics; D046911:Macromolecular Substances; D008870:Microtubules / Q000032:analysis; D009994:Osmolar Concentration; D010063:Ovum / Q000032:analysis*; D011485:Protein Binding; D012617:Sea Urchins; D013552:Swine; D014404:Tubulin / Q000302:isolation & purification
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D046911:Macromolecular Substances; D014404:Tubulin; D002118:Calcium