Microtubules are highly dynamic polymers built from the addition and loss of tubulin dimers at their ends. Microtubule dynamics are essential to achieve fundamental cellular processes such as cell division. In vivo, microtubule assembly is tightly regulated. This regulation mainly results from the activity of microtubule effectors that interact with microtubules or tubulin dimers. There are two main types of microtubule effectors, those that favour the polymerised state of microtubules (mostly MAPs), and those that promote microtubule depolymerisation. Katanin is the best described microtubule-severing protein. Animal katanin is a hetero-dimer protein, composed of a catalytic subunit of 60 kDa (P60) and a regulatory subunit of 80 kDa (P80). P60 is an AAA (ATPases associated with various cellular activities) protein which hydrolyses ATP in a microtubule-dependent manner and is sufficient to sever microtubules in vitro (McNally et al., 2000). In animal cells, katanin is thought to be involved in the release of microtubules from centrosomes and the regulation of the number of microtubule ends in the spindle (Buster et al., 2002). In higher plants, no homologue of the P80 katanin regulatory subunit has been described so far. Recently two Arabidopsis thaliana mutants mutated in a gene showing significant homologies with the animal P60 catalytic katanin subunit have been described (Bichet et al., 2001; Burk et al., 2001; for a description of katanin mutants, see the abstract of D. Bouchez on the BOTERO mutant). Surprisingly, the microtubule cytoskeleton of these mutants has only very subtle defects, and cells seem to divide normally. Molecular effects of the plant P60 on microtubule assembly were not known. To study microtubule destabilization in plant cells, we cloned the cDNA encoding for the P60 orthologue in A. thaliana (AtP60) and functionally characterized the properties of a recombinant His-tagged AtP60 (Stoppin-Mellet et al., 2002). Using both video-microscopy assays and spectrofluorimetry, we showed for the first time that HisAtP60 can sever microtubules in vitro in the presence of ATP. His-AtP60 directly interacts with microtubules in co-sedimentation assays. In the presence of microtubules, the ATPase activity of AtP60 was stimulated in a non-hyperbolic way. The basal ATPase activity of AtP60 (calculated at one molecule ATP/AtP60/sec) was stimulated up to six times at low AtP60/tubulin molar ratio (maximum at 0.04), and inhibited at higher ratios. AtP60 is the first plant protein shown to fragment microtubules. The characterization of the functional domains of AtP60 is now under way, both in vitro and in vivo.