Posttranslational processing of alpha-tubulin during axoplasmic transport in CNS axons.

Abstract

Tubulin proteins in mouse retinal ganglion cell (RGC) neurons were analyzed to determine whether they undergo posttranslational processing during axoplasmic transport, .alpha.- and .beta.-tubulin comprised heterogeneous proteins in the primary optic pathway (optic nerve and optic tract) when examined by 2-dimensional (2D) PAGE [polyacrylamide gel electrophoresis]. .alpha.-Tubulin exhibited regional heterogeneity when consecutive 1.1-mm segments of the optic pathway were analyzed separately. In proximal segments, .alpha.-tubulin consisted of 2 predominant proteins separable by isoelectric point and several less abundant species. In more distal segments, these predominant proteins decreased progressively and the .alpha.-tubulin region of the gel was represented by less abundant multiple forms only; .beta.-tubulin was the same in all segments. After intravitreal injection of [3H]proline to mice, radiolabeled a- and .beta.-tubulin heteroproteins were conveyed together at a rate of 0.1-0.2 mm/d in the slowest plase of axoplasmic transport. At 45 d postinjection, the distribution of radiolabeled heterogeneous forms of .alpha.- and .beta.-tubulin in consecutive segments of optic pathway resembled the distribution of unlabeled proteins by 2D PAGE, indicating that regional heterogeneity of tubulin arises during axonal transport. Peptide mapping studies demonstrated that the progressive alteration of .alpha.-tubulin revealed by PAGE analysis cannot be explained by contamination of the .alpha.-tubulin region by other proteins on gels. The results are consistent with the posttranslational processing of .alpha.-tubulin during axoplasmic transport. These observations provide additional evidence that CNS axons may be regionally specialized.