MICROTUBULE RELEASE FROM THE CENTROSOME

Although microtubules (MTs) are generally thought to originate at the centrosome, a number of cell types have significant populations of MTs with no apparent centrosomal connection, The origin of these noncentr osomal MTs has been unclear, We applied kinetic analysis of MT formati on in vivo to establish their mode of origin, Time-lapse fluorescence microscopy demonstrated that noncentrosomal MTs in cultured epithelial cells arise primarily by constitutive nucleation at, and release from , the centrosome. After release, MTs moved away from the centrosome an d tended to depolymerize. Laser-marking experiments demonstrated that released MTs moved individually with their plus ends leading, suggesti ng that they were transported by minus end-directed motors, Released M Ts were dynamic, The laser marking experiments demonstrated that plus ends of released MTs grew, paused, or shortened while the minus ends w ere stable or shortened, Microtubule release may serve two kinds of ce llular function, Release and transport could generate the noncentrosom al MT arrays observed in epithelial cells, neurons, and other asymmetr ic, differentiated cells, Release would also contribute to polymer tur nover by exposing MT minus ends, thereby providing additional sites fo r loss of subunits, The noncentrosomal population of MTs may reflect a steady-state of centrosomal nucleation, release, and dynamics.