Calcium lability of cytoplasmic microtubules and its modulation by microtubule-associated proteins

Detergent-extracted BSC-1 monkey cells have been used as a model system to study the Ca(2+) sensitivity of in vivo polymerized microtubules under in vitro conditions. The effects of various experimental treatments were observed by immunofluorescence microscopy. Whereas microtubules are completely stable at Ca(2+) concentrations below 1 muM, Ca(2+) at greater than 1-4 muM induces microtubule disassembly that begins in the cell periphery and proceeds towards the cell center. At concentrations of up to 500 muM, both the pattern and time course of disassembly are not markedly altered, suggesting that, within this concentration range, Ca(2+) effects are catalytic rather than stoichiometric. Higher (millimolar) Ca(2+) concentration results in rapid destruction of microtubules. Of other divalent cations, only Sr(2+) has a slight depolymerizing effect, whereas millimolar Ba(2+), Mg(2+), or Mn(2+) is ineffective. Disassembly induced by micromolar Ca(2+) is inhibited by pharmacological agents known to bind to calmodulin and inhibit its function, suggesting that calmodulin mediates Ca(2+) effects. Both the addition of exogenous brain microtubule-associated proteins (MAPs) after lysis and the retention of endogenous cellular MAPs normally extracted during the lysis step stabilize microtubules against the depolymerizing effect of micromolar Ca(2+). The results indicate that, in this model system, microtubules are sensitive to physiological Ca(2+) concentrations and that this sensitivity may be conferred by calmodulin associated with the microtubules. MAPs appear to have a modulating effect on microtubular Ca(2+) sensitivity and thus may function as a discriminating factor in cellular functions performed by calmodulin. It is hypothesized that Ca(2+)-stimulated microtubule disassembly depends on the relative amount of MAPs.