Depolymerization of microtubules resulted in an increase in the motional freedom of molecular probes in the plasma membranes of Chinese hamster ovary cells expressed by the order parameter, S, measured with two different lipid-soluble spin label probes, 5-doxyl stearic acid and 16-doxyl methylstearate. Treatment with a variety of microtubule-depolymerizing agents, including Colcemid, colchicine, vinblastine, podophyllotoxin, and griseofulvin, all had similar effects on motional freedom of the probes whereas beta-lumicolchicine was inactive. Several independent lines of evidence suggest that these changes in motional freedom of the probes were not the direct result of the interaction of these relatively hydrophobic drugs with the plasma membrane: the effects of the drugs were not immediate; the dose response of the Colcemid effect was the same as the dose response for depolymerization of microtubules; taxol, which stabilizes microtubules but does not affect motional freedom in the membranes, blocked the effect of Colcemid on motional freedom; a mutant cell line which is resistant to colchicine because of reduced uptake of the drug showed no effects of colchicine on probe motional freedom; and a Colcemid-resistant mutant cell line with an altered beta-tubulin showed no effect of Colcemid on motional freedom in the membrane. These results support the hypothesis that microtubules might affect, directly or indirectly, plasma membrane functions.

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