Microtubule stability is known to be governed by a stabilizing GTP/GDP-Pi cap, but the exact relation between growth velocity, GTP hydrolysis, and catastrophes remains unclear. We investigate the dynamics of the stabilizing cap through in vitro reconstitution of microtubule dynamics in contact with microfabricated barriers, using the plus-end binding protein GFP-EB3 as a marker for the nucleotide state of the tip. The interaction of growing microtubules with steric objects is known to slow down microtubule growth and accelerate catastrophes. We show that the lifetime distributions of stalled microtubules, as well as the corresponding lifetime distributions of freely growing microtubules, can be fully described with a simple phenomenological 1D model based on noisy microtubule growth and a single EB3-dependent hydrolysis rate. This same model is furthermore capable of explaining both the previously reported mild catastrophe dependence on microtubule growth rates and the catastrophe statistics during tubulin washout experiments.

Thin films of uranium cerium mixed oxides U_xCe_1−xO_2±y have been prepared by DC sputtering and characterized by X-ray photoelectron spectroscopy (XPS). Reduction and oxidation properties were analysed by exposing the films to atomic hydrogen and to atomic oxygen, respectively. Possible interaction between uranium and cerium were investigated by comparing the compounds to the single oxides of cerium and uranium. Uranium seems to promote the reduction of cerium. Conversely cerium promotes oxidation of uranium. Under our deposition conditions, it was not possible to produce pure U(IV) together with pure Ce(IV). The synthesis of U₂O₅ with U(V) cannot be obtained directly by DC sputtering, whereas with addition of cerium, U(V) seems to be stabilized and obtained more easily. This can be related to a charge transfer between Ce(IV) and U(IV).