DIFFERENCES IN THE EFFECT OF CA2+ ON ISOLATED MICROTUBULES FROM COD AND COW BRAIN

Isolated microtubules from cod and cow brains were compared with respe ct to their response to calcium ions. The effect of Ca2+ on cod microt ubules was found to be temperature dependent. In contrast to cow micro tubules, cod microtubules assembled at 18 degrees C. At this temperatu re the assembly was inhibited by Ca2+ concentrations of 2 mM and highe r. This was also found for cow microtubules at 37 degrees C. However, at 30 degrees C there was no effect of 2 mM Ca2+ of the amount of asse mbly or disassembly of cod microtubules consisting of only tubulin or of tubulin and microtubule-associated proteins (MAPs). The morphology was affected though, since some coiled ribbons formed from tubulin and MAPs. The calcium-binding calmodulin did not alter the effect of calc ium on cod microtubules markedly. At higher Ca2+ concentrations (> 4 m M), coiled ribbons were formed from cod tubulin and MAPs, but mainly a morphous aggregates and very few coiled ribbons were formed from cod t ubulin alone, indicating that the Ca2+ effect is modulated by cod MAPs . The modulatory effect of cod MAPs was however not species specific, since both cod and cow MAPs had the same effect on cod microtubules, i n spite of a different protein composition. A MAP-dependent effect of Ca2+ was also found for cow microtubule proteins. The assembly of pure cow tubulin, as well as that of cow tubulin and MAPs, was inhibited b y 2 mM Ca2+. In the presence of 10 and 20 mM Ca2+, pure cow tubulin fo rmed amorphous aggregates, rings, and even paracrystals, while the ass embly of cow tubulin and MAPs was inhibited. Our results suggest there fore that the effect of Ca2+ can be moderated by MAPs, but depends on intrinsic properties of the different tubulins. (C) 1994 Wiley-Liss, I nc.