Dynamic properties of microtubules at steady state of polymerisation

Microtubules of tubulin dimer, polymerised in vitro to steady-state are shown to incorporate tubulin rapidly and extensively. The method involves adding [3H]-GTP to microtubules at steady state, and analysing for non-exchangeable [3H]-GDP in the presence of a GTP regenerating system. The rate and extent of this exchange process is dependent on the length distribution of the microtubules, and is notably faster with sheared microtubules. We simulate all these features of the exchange kinetics, together with the length redistributions occurring at steady state of polymerisation, using a simple model based on a limited number of kinetic parameters deriving from the measurements of microtubule dynamics by Horio and Hotani. The observed exchange kinetics thus provide a direct experimental criterion of 'dynamic instability' of microtubules at steady state of polymerisation.