GAMMA-TUBULIN CAN BOTH NUCLEATE MICROTUBULE ASSEMBLY AND SELF-ASSEMBLE INTO NOVEL TUBULAR STRUCTURES IN MAMMALIAN-CELLS

alpha-, beta-, and gamma-tubulins are evolutionarily highly conserved members of the tubulin gene superfamily. While the abundant members, a lpha- and beta-tubulins, constitute the building blocks of cellular mi crotubule polymers, gamma-tubulin is a low abundance protein which loc alized to the pericentriolar material and may play a role in microtubu le assembly. To test whether gamma-tubulin mediates the nucleation of microtubule assembly in vivo, and co-assembles with alpha- and beta-tu bulins into microtubules or self-assembles into macro-molecular struct ures, we experimentally elevated the expression of gamma-tubulin in th e cell cytoplasm. In most cells, overexpression of gamma-tubulin cause s a dramatic reorganization of the cellular microtubule network. Furth ermore, we show that when overexpressed, gamma-tubulin causes ectopic nucleation of microtubules which are not associated with the centrosom e. In a fraction of cells, gamma-tubulin self-assembles into novel tub ular structures with a diameter of similar to 50 nm (named gamma-tubul es). Furthermore, unlike microtubules, gamma-tubules are resistant to cold or drug induced depolymerization. These data provide evidence tha t gamma-tubulin can cause nucleation of microtubule assembly and can s elf-assemble into novel tubular structures.