J Biochem 1980 Jan 01;871:143-51. doi: 10.1093/oxfordjournals.jbchem.a132719.

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.

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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.

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Genes referenced: LOC100887844 tubgcp2