α-Tubulin is not detyrosylated during axonal transport

We have examined the question of whether α-tubulin is detyrosylated during axonal transport in retinal ganglion cell axons and axons of spinal motor neurons. The degree of tyrosylation of α-tubulin was estimated from immunocytochemistry and immunoblotting with two anti-α-tubulin monoclonals, one of which (YL1/2) recognizes only the tyrosylated form of α-tubulin. In the case of retinal ganglion cells, the axons were depleted of tyrosylated α-tubulin both in the retina and proximal region of the optic nerve. Distal regions of the axons, in the optic tract, gave a pattern of staining consistent with a reduction in the total level of α-tubulin at the expense of detyrosylated α-tubulin. Axons within the L5 ventral root, the sciatic nerve and tibial nerve were consistently unstained by YL1/2 indicating that these axons were depleted in tyrosylated α-tubulin in all 3 segments. The results indicate that α-tubulin destined for axonal microtubules is detyrosylated close to or in cell bodies and not progressively during its transport. Therefore the segregation of detyrosylated α-tubulin to axonal microtubules may occur at their site of assembly.