Subunit composition specific to axonally transported tubulin.

One week after injection of L-[35S]methionine into the dorsal motor nuclei of the guinea-pig, labelled tubulin carried down the vagal nerve by the slow phase of axonal transport was analysed by one- and two-dimensional gel electrophoresis. Transported tubulin showed a much stronger labelling of the beta-subunit. Isoelectric focussing revealed that both alpha- and beta-subunits were composed of several components. Labelled tubulin was isolated from the brain by cycles of polymerisation and depolymerisation after injection of L-[35S]methionine into the lateral ventricle, for comparison with transported tubulin from the vagal nerve. In addition to the two alpha-components and three beta-components detected in both preparations, axonally transported tubulin contained an extra component (TAX) with a molecular weight corresponding to that of beta-tubulin and with the same isoelectric point as alpha-tubulin. The axon-specific component TAX co-polymerised with tubulin isolated from the brain. Upon peptide mapping by limited proteolysis, the peptide pattern generated from TAX was similar to that of the alpha-tubulin. It is concluded that the axonally transported tubulin contains a modified alpha-subunit which is not found in the bulk of brain tubulin.