Stabilization of microtubules by tubulin-GDP-Pi subunits

Microtubule dynamic instability has been accounted for by assuming that tubulin subunits at microtubule ends differ from the tubulin-GDP subunits that constitute the bulk of the microtubule. It has been suggested that this heterogeneity results because ends contain tubulin subunits that have not yet hydrolyzed an associated GTP molecule. Alternatively, in a recent model it was proposed that ends contain tubulin-GDP-Pi subunits from which Pi has not yet dissociated. The models differ in their predicted response to added ligands: because GDP in subunits in microtubules does not exchange with nucleotide in solution, the heterogeneity from a tubulin-GTP cap will not be eliminated by added GTP; however, the dissociability of Pi in tubulin-GDP-Pi subunits will allow a heterogeneity resulting from a tubulin-GDP-Pi cap to be eliminated by added excess Pi. Elimination of the heterogeneity is expected to be manifested by an elimination of dynamic instability behavior. Using video microscopy to study the kinetic behavior of individual microtubules under reaction conditions where dynamic instability is the dominant mechanism for microtubule length changes, we have determined the effects of 0.167 M Pi on the rate of subunit addition in the elongation phase, the rate of subunit dissociation in the rapid shortening phase, and the rates of the phase transitions from elongation to rapid shortening and from rapid shortening to growing. Since 0.167 M Pi did not decrease the subunit dissociation rate in the rapid shortening phase or the rate of the phase transition from growing to rapid shortening, our results provide no support for the hypothesis that tubulin-GDP-Pi subunits are responsible for dynamic instability behavior of microtubules.(ABSTRACT TRUNCATED AT 250 WORDS)