R. Takemura et al., POLARITY ORIENTATION AND ASSEMBLY PROCESS OF MICROTUBULE BUNDLES IN NOCODAZOLE-TREATED, MAP2C-TRANSFECTED COS CELLS, Molecular biology of the cell, 6(8), 1995, pp. 981-996
Microtubule bundles reminiscent of those found in neuronal processes a
re formed in fibroblasts and Sf9 cells that are transfected with the m
icrotubule-associated proteins tau, MAP2, or MAP2c. To analyze the ass
embly process of these bundles and its relation to the microtubule pol
arity, we depolymerized the bundles formed in MAP2c-transfected COS ce
lls using nocodazole, and observed the process of assembly of microtub
ule bundles after removal of the drug in cells microinjected with rhod
amine-labeled tubulin. Within minutes of its removal, numerous short m
icrotubule fragments were observed throughout the cytoplasm. These sho
rt fragments were randomly oriented and were already bundled. Somewhat
longer, but still short bundles, were then found in the peripheral cy
toplasm. These bundles became the primordium of the larger bundles, an
d gradually grew in length and width. The polarity orientation of micr
otubules in the reformed bundle as determined by ''hook'' procedure us
ing electron microscope was uniform with the plus end distal to the ce
ll nucleus. The results suggest that some mechanism(s) exists to orien
t the polarity of microtubules, which are not in direct continuity wit
h the centrosome, during the formation of large bundles. The observed
process presents a useful model system for studying the organization o
f microtubules that are not directly associated with the centrosomes,
such as those observed in axons.