MODULATION OF MICROTUBULE DYNAMIC INSTABILITY IN-VIVO BY BRAIN MICROTUBULE-ASSOCIATED PROTEINS

Heat-stable brain microtubule associated proteins (MAPs) and purified microtubule associated protein 2 (MAP-2) were microinjected into cultu red BSC-1 cells which had been previously injected with rhodamine-labe led tubulin. The dynamic instability behavior of individual microtubul es was then examined using low-light-level fluorescence microscopy and quantitative microtubule tracking methods, Both MAP preparations supp ressed microtubule dynamics in vivo, by reducing the average rate acid extent of both growing and shortening events, The average duration of growing events was not affected, When measured as events/unit time, h eat-stable MAPs and MAP-2 did not significantly alter the frequency of rescue; the frequency of catastrophe was decreased approximately two- fold by heat-stable MAPs and MAP-2, When transition frequencies were c alculated as events/unit distance, both MAP preparations increased the frequency of rescue, without altering the frequency of catastrophe, T he percentage of total time spent in the phases of growth, shrink and pause was determined, Both MAP-2 and heat-stable MAPs decreased the pe rcentage of time spent shortening, increased the percentage of time sp ent paused, and had no effect on percentage of time spent growing, Hea t-stable MAPs increased the average pause duration, decreased the aver age number of events per minute per microtubule and increased the prob ability that a paused microtubule would switch to growing rather than shortening. The results demonstrate that addition of MAPs to living ce lls reduces the dynamic behavior of individual microtubules primarily by suppressing the magnitude of dynamic events and increasing the time spent in pause, where no change in the microtubule length can be dete cted. The results further suggest that the expression of MAPs directly contributes to cell type-specific microtubule dynamic behavior.