IO

Although considerable in girth (25 nm) and held together by multiple tubulin–tubulin interactions, microtubules have been observed to “snap ” in half in various cell types (Waterman-Storer and Salmon, 1997; Kalil et al., 2000). The fi rst evidence that microtubule breakage refl ects the action of an enzyme rather than just mechanical strain came from experiments with Xenopus laevis egg mitotic extracts (Vale, 1991). Fittingly, this microtubule-severing enzyme (which consists of a catalytic subunit, p60, belonging to the AAA ATPase family and a regu-latory subunit, p80) was named katanin, after the Japanese word for sword, katana (McNally and Vale, 1993). Katanin is found in a variety of organisms ranging from Chlamydomonas reinhardtii to humans, but its biological roles remain poorly understood. However, two recent studies signifi cantly advance our understanding of katanin-mediated microtubule severing