Tubulin domains responsible for assembly of dimers and protofilaments.

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Abstract
The protein domains responsible for the dimerization and polymerization of tubulin have been determined using chemical cross‐linking and limited proteolysis. The intra‐dimer bond is formed by the N‐terminal domain of alpha‐tubulin and the C‐terminal domain of beta‐tubulin. Conversely, the inter‐dimer bond along protofilaments is formed by the N‐terminal domain of beta‐tubulin (carrying the exchangeable GTP) and the C‐terminal domain of alpha‐tubulin. The domains of proteolytically cleaved tubulin remain tightly associated in solution. Apart from the monomer, tubulin shows three levels of assembly: the dimer, oligomer and polymer. Several oligomeric species can be visualized by electron microscopy of rotary shadowed phosphocellulose‐tubulin, h.p.l.c. and non‐denaturing gel electrophoresis. Tubulin's capacity to form the higher level aggregates is not destroyed by enzymatic nicking.