The dynamic distribution of fluorescent analogues of actin and myosin in protrusions at the leading edge of migrating Swiss 3T3 fibroblasts

The formation of protrusions at the leading edge of the cell is an essential step in fibroblast locomotion. Using fluorescent analogue cytochemistry, ratio imaging, multiple parameter analysis, and fluorescence photobleaching recovery, the distribution of actin and myosin was examined in the same protrusions at the leading edge of live, locomoting cells during wound-healing in vitro. We have previously defined two temporal stages of the formation of protrusions: (a) initial protrusion and (b) established protrusion (Fisher et al., 1988). Actin was slightly concentrated in initial protrusions, while myosin was either totally absent or present at extremely low levels at the base of the initial protrusions. In contrast, established protrusions contained diffuse actin and actin microspikes, as well as myosin in both diffuse and structured forms. Actin and myosin were also localized along concave transverse fibers near the base of initial and established protrusions. The dynamics of myosin penetration into a relatively stable, established protrusion was demonstrated by recording sequential images over time. Myosin was shown to be absent from an initial protrusion, but diffuse and punctate myosin was detected in the same protrusion within 1-2 min. Fluorescence photobleaching recovery indicated that myosin was 100% immobile in the region behind the leading edge containing transverse fibers, in comparison to the 21% immobile fraction detected in the perinuclear region. Possible explanations of the delayed penetration of myosin into established protrusions and the implications on the mechanism of protrusion are discussed.