The formation of macrotubules by the volatile anaesthetic halothane was investigated in vitro using microtubule-enriched fractions of crayfish nerve cords. Sequential studies showed that macrotubules assemble from helical ribbons of 18–20 laterally associated microtubule protofilaments which fold upon themselves to form intact macrotubules averaging 48 nm in diameter. The initial rate of macrotubule assembly is dependent on the concentration of halothane employed and is stimulated by calcium. Glycerol pretreatment blocked macrotubule formation by halothane and caused preformed macrotubules to reassemble rapidly into typical microtubules. These experiments show that microtubules and macrotubules require different conditions for assembly and support the contention that macrotubule formation by halothane is due to a direct interaction between the anaesthetic molecule and the microtubule subunit.

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