Possible translocation of actin and alpha-actinin along stress fibers

We have employed fluorescent analogue cytochemistry and fluorescence photobleaching to study the mobility of actin and alpha-actin along stress fibers. Rhodamine-labeled actin or alpha-actinin microinjected into embryonic chick cardiac fibroblasts soon became incorporated into stress fibers. A pulse of a laser microbeam was used to photobleach small spots on the fluorescent stress fibers. Images of the bleached fiber were recorded with an intensified image processing system at 2-3 min intervals. The distance between the bleached spot and the terminus of the stress fiber, which remained stationary throughout the experiment, was then measured in the successive images. Movement of bleached spots was detected along stress fibers located in the apparently trailing processes of polygonal fibroblasts, and only occurred in one direction: away from the distal tip of the stress fiber. The rate of movement calculated for alpha-actinin-injected cells was 0.24 +/- 0.12 micron/min, for actin-injected cells, 0.29 +/- 0.11 micron/min. The rate did not seem to be affected by the location of the spot relative to the distal end of the stress fiber unless the spot was located within the most distal 5 microns of the stress fiber. Anti-myosin antibody staining indicated that stress fibers which demonstrated translocation were relatively depleted of myosin. The apparent translocation of proteins along stress fibers, possibly generated by stretching, may be related to the retraction of cell processes during locomotion.