Calcium Lability of Cytoplasmic Microtubules and Its Modulation by Microtubule-Associated Proteins

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 78; no. 2; pp. 1037 - 1041
• Main Authors: Schliwa, Manfred, Euteneuer, Ursula, Bulinski, Jeannette Chloe, Izant, Jonathan G.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01-02-1981; National Acad Sciences
• Subjects: Animals; Biological Sciences: Cell Biology; Calcium - pharmacology; Calmodulin - pharmacology; Cell extracts; Cells; Cercopithecus aethiops; Depolymerization; Detergents; Fluorescent Antibody Technique; Fluorescent antibody techniques; Glycols; HeLa cells; Kidney; Microscopy; Microtubule-Associated Proteins; Microtubules; Microtubules - drug effects; Microtubules - ultrastructure; Nerve Tissue Proteins - metabolism; Physiological regulation; Proteins - metabolism
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.78.2.1037

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 immuno-fluorescence microscopy. Whereas microtubules are completely stable at Ca2+concentrations below 1 μ M, Ca2+at greater than 1-4 μ M induces microtubule disassembly that begins in the cell periphery and proceeds towards the cell center. At concentrations of up to 500 μ M, 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.