Mechanisms of assembly and disassembly of microtubules.

5.3-nm skewed sur face lattice. They can be considered to be made of 13 nearly axial proto filaments. Tubulin is a non-covalently linked dimer of partially homol ogous but non­ ide ntica l chains (a: and {J) of approximately 53,000 mol wt and of-almost fully determined amino acid sequence. Tubulin from a single tissue is heterogeneous, and there are at least 17 kinds of IX-chains and {J-chains together in preparations from mammalian brain. Microtubu les assemble spon taneously in the presence of GTP and Mg2 + at temperatures near 37°C and disassemble at temperatures near 4°C. A common isolation technique is to cause micro tubules to assemble in an extract of