Assembly of purified brain tubulin at cortical and endoplasmic sites in perfused internodal cells of the alga Nitella tasmanica

We have identified sites in the alga, Nitella tasmanica, that promote the assembly of exogenous brain tubulin. Biotinylated brain tubulin was perfused into giant internodal cells in buffers that did not support polymerization in vitro. Exogenous tubulin — detected with streptavidin — bound to nuclei and polymerized in the endoplasm to give microtubules that often terminated on nuclei. Microtubule assembly in the cortical cytoplasm was very limited unless there was damage to the chloroplast array separating the introduced tubulin from its normal cortical assembly sites adjacent to the plasma membrane. Microtubules then assembled in large numbers, often radiating out from small foci. Similar nuclear and cortical sites have been found to initiate the in vivo reassembly of Nitella microtubules following experimental depolymerisation.