We labeled proteins in the cell bodies of rabbit retinal ganglion cells with [35S]methionine and subsequently observed the appearance of radioactive actin in tissues containing the axons and synaptic terminals of these neurons, i.e., the optic nerve (ON), optic tract (OT), lateral geniculate nucleus (LGN) and the superior colliculus (SC). The temporal sequence of appearance of labeled actin (which was identified by its specific binding to DNase I, its electrophoretic mobility, and its peptide map) in these tissues indicated that actin is an axonally transported protein with a maximum transport velocity of 3.4--4.3 mm/d. The kinetics of labeling actin were similar to the kinetics of labeling two proteins (M1 and M2) which resemble myosin; these myosin-like proteins were previously found to be included in the groups of proteins (groups III and IV) transported with the third and fourth most rapid maximum velocities. The similarity in transport between actin and myosin-like proteins supports the idea that a number of proteins in the third and fourth transport groups may be functionally related by virtue of their involvement in a force-generating mechanism and suggests the possibility that these proteins may be axonally transported as a preformed force-generating unit.

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