Concerning the chemical nature of tubulin subunits that cap and stabilize microtubules.

There is no definitive evidence on the nature of the cap at microtubule ends that is responsible for dynamic instability behavior. It was, therefore, of interest that steady-state microtubules assembled in 20 mM Pi buffer and pulsed for 15−60 min with [γ-32P]GTP contained approximately 26 [32P]Pi/microtubule [Panda et al. (2002) Biochemistry 41, 1609−1617]. It was concluded that microtubules are capped with a tubulin−GDP−Pi subunit at the end of each its 13 protofilaments and that this is responsible for stabilizing microtubules in the growth phase. Also, because microtubules with [32P]Pi were isolated despite the presence of 20 mM Pi, it was concluded that Pi in terminal tubulin−GDP−Pi subunits does not exchange with solvent. These observations are inconsistent with our finding that tubulin−GDP−Pi subunits do not stabilize microtubules and with evidence that the nucleotide, and presumably also Pi, in subunits at microtubule ends exchanges with solvent. We have resolved this discrepancy by finding that du...