Polyamine Sharing between Tubulin Dimers Favours Microtubule Nucleation and Elongation via Facilitated Diffusion

We suggest for the first time that the action of multivalent cations on microtubule dynamics can result from facilitated diffusion of GTP-tubulin to the microtubule ends. Facilitated diffusion can promote microtubule assembly, because, upon encountering a growing nucleus or the microtubule wall, random GTP-tubulin sliding on their surfaces will increase the probability of association to the target sites (nucleation sites or MT ends). This is an original explanation for understanding the apparent discrepancy between the high rate of microtubule elongation and the low rate of tubulin association at the microtubule ends in the viscous cytoplasm. The mechanism of facilitated diffusion requires an attraction force between two tubulins, which can result from the sharing of multivalent counterions. Natural polyamines (putrescine, spermidine, and spermine) are present in all living cells and are potent agents to trigger tubulin self-attraction. By using an analytical model, we analyze the implication of facilitated diffusion mediated by polyamines on nucleation and elongation of microtubules. In vitro experiments using pure tubulin indicate that the promotion of microtubule assembly by polyamines is typical of facilitated diffusion. The results presented here show that polyamines can be of particular importance for the regulation of the microtubule network in vivo and provide the basis for further investigations into the effects of facilitated diffusion on cytoskeleton dynamics

Riparian wetlands of the middle reach of the Seine river (France): historical development, investigation and present hydrologic functioning. A case study

International audienceAlong the middle reach of the Seine river, upstream from Paris, riparian wetlands that have been functioning for centuries, first as a result of a succession of sedimentological/hydrological processes and later of continuous management practices where the value of the wetlands has been acknowledged as well as their importance for the ecology. Here, we present the results of a multidisciplinary research programme focused on this system with emphasis on its present hydrological functioning and on the tools used to understand and quantify it. The major hydrological and geochemical processes that are active at present are reviewed, but past processes are also described as well as the potential long-term evolution of the system. This study uses several innovative tools to investigate the wetlands and analyse them in space: thermographic aerial survey and electromagnetic prospecting resulting in a map of clay-layer thickness. Concerning the wetland hydrology, water budgets, established at different time scales (several years, one year, a season) illustrate the exchanges between the watershed, the river and the wetland and water storage in the wetlands during flood events. The results show how the upstream reservoirs, built during the last 20 years, presently control the preservation and functioning of downstream riparian wetlands. Monitoring and modelling of the hydrological functioning of one wetland site have made it possible to identify and quantify exchanges between the wetland (especially in the unsaturated upper layer) and its environment. In situ measurements of denitrification rates in the wetland were used to make initial estimates of its nitrate-elimination capacity