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

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Detergent-extracted BSC-1 monkey cells have been used as a model system to study the Ca2+ 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 Ca2+ concentrations below 1 μM, Ca2+ at greater than 1-4 μM induces microtubule disassembly that begins in the cell periphery and proceeds towards the cell center. At concentrations of up to 500 μM, both the pattern and time course of disassembly are not markedly altered, suggesting that, within this concentration range, Ca2+ effects are catalytic rather than stoichiometric. Higher (millimolar) Ca2+ concentration results in rapid destruction of microtubules. Of other divalent cations, only Sr2+ has a slight depolymerizing effect, whereas millimolar Ba2+, Mg2+, or Mn2+ is ineffective. Disassembly induced by micromolar Ca2+ is inhibited by pharmacological agents known to bind to calmodulin and inhibit its function, suggesting that calmodulin mediates Ca2+ 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 Ca2+. The results indicate that, in this model system, microtubules are sensitive to physiological Ca2+ concentrations and that this sensitivity may be conferred by calmodulin associated with the microtubules. MAPs appear to have a modulating effect on microtubular Ca2+ sensitivity and thus may function as a discriminating factor in cellular functions performed by calmodulin. It is hypothesized that Ca2+-stimulated microtubule disassembly depends on the relative amount of MAPs.

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