Axonal transport of actin: Slow component b is the principal source of actin for the axon

Abstract

Axonally transported proteins were studied in guinea pig retinal ganglion cells using the standard radioisotopic labeling procedure. Two slowly moving groups of proteins were identified in guinea pig retinal ganglion cells. The more slowly moving group of proteins, designated slow component a (SCa) was transported at 0.2–0.5 mm/day. Five polypeptides contained greater than 75% of the total radioactivity transported in SCa. Two of these polypeptides correspond to the subunits of tubulin, while the other three correspond to the slow component triplet 16,19. The other slowly moving group of proteins, which is designated slow component b (SCb), was transported at approximately 2 mm/day. Twenty labeled polypeptides were identified in SCb. The major labeled polypeptides transported in SCb differ from those transported in SCa. One of the polypeptides transported in SCb co-migrates with skeletal muscle actin in SDS-polyacrylamide slab gels. This polypeptide behaved identically to skeletal muscle actin on DNaseI affinity columns. Since DNaseI is a highly specific affinity ligand for actin 21, we conclude that the labeled SCb polypeptide which co-migrates with actin in SDS-gels is actin. Between 1.4 and 5.7% of the total radioactivity transported in SCb is attributable to actin. Detailed comparison of the distribution of total radioactivity in the optic axons with the distribution of radioactive actin in the optic axons at post-injection times between 6 and 77 days showed that actin was transported specifically in SCb, and not in SCa. Furthermore, analyses of the proteins transported in the fast component of guinea pig retinal ganglion cells by DNaseI affinity chromatography failed to reveal an actin-like moiety. Slow component a, SCb and the fast component are the major components of axonal transport in guinea pig retinal ganglion cells. Thus, in these neurons, actin is transported principally and possibly only in SCb. Guinea pig retinal ganglion cell axons project principally to the lateral geniculate nucleus and superior colliculus. The fate of actin axonally transported to the region of the axon terminals was studied by determining the kinetics by which radioactivity associated with actin accumulates and then decays in the superior colliculus. The results of these studies indicate that labeled actin has a half-life in the superior colliculus of approximately 28 days.