SLOW AXONAL-TRANSPORT OF SOLUBLE ACTIN WITH ACTIN DEPOLYMERIZING FACTOR, COFILIN, AND PROFILIN SUGGESTS ACTIN MOVES IN AN UNASSEMBLED FORM

We examined the axonal transport of actin and its monomer binding prot eins, actin depolymerizing factor, cofilin, and profilin, in the chick en sciatic nerve following injection of [S-35]methionine into the lumb ar spinal cord. At intervals up to 20 days after injection, nerves wer e cut into 1-cm segments and separated into Triton X-100-soluble and p articulate fractions. Actin and its binding proteins were then isolate d by affinity chromatography on DNase I-Sepharose and by one- and two- dimensional polyacrylamide gel electrophoresis, Fluorographic analysis showed that the specific activity of soluble actin was two to three t imes that of its particulate form and that soluble actin, cofilin, act in depolymerizing factor, and profilin were transported at similar rat es in slow component b of axonal flow. Our data strongly support the v iew that the mobile form of actin in slow transport is soluble and tha t a substantial amount of this actin may travel as a complex with acti n depolymerizing factor, cofilin, and profilin. Along labeled nerves t he specific activity of the unphosphorylated form of actin depolymeriz ing factor, which binds actin, was not significantly different from th at of its ''inactive'' phosphorylated form. This constancy in specific activity suggests that continuous inactivation and reactivation of ac tin depolymerizing factor occur during transport, which could contribu te to the exchange of soluble actin with the filamentous actin pool.