Biochemical assay of microtubule mean length.

We present a method for the rapid determination of microtubule mean length in vitro. This method rests on mathematical analysis of the rate of polymer disassembly induced by the introduction of calcium at a known concentration. The rate of disassembly is monitored in our assay by filter trapping of residual microtubule polymers, which contain a radioactive tracer, [3H]GTP. We show that the assay is accurate and reproducible, by comparison with physical measurement of lengths from electron micrographs. Furthermore, we show that the assay can be used to determine rapid shifts in polymer length induced in polymer populations that exhibit "dynamic instability".