The Journal of Cell Biology, Vol 94, 159-164, Copyright © 1982 by The Rockefeller University Press
Posttranslational processing of alpha-tubulin during axoplasmic transport in CNS axons
BA Brown, RA Nixon and CA Marotta
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 two-dimensional (2D) PAGE. In addition, however, 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 two 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 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 alpha- and beta-tubulin heteroproteins
were conveyed together at a rate of 0.1-0.2 mm/d in the slowest phase of
axoplasmic transport. At 45 d postinjection, the distribution of
radiolabeled heterogeneous forms a 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, along with
the accompanying report (J. Cell Biol., 1982, 94:150-158), provide
additional evidence that CNS axons may be regionally specialized.
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