Actin accumulation in pseudopods or in the tail of polarized walker carcinosarcoma cells quantitatively correlates with local folding of the cell surface membrane.

We determined the actin distribution and the relationship between actin and the cell surface membrane in polarized Walker carcinosarcoma cells showing lamellipodia or blebs at the front in order to get a better insight into actin's role in shape changes and cell locomotion. Using two different techniques, we found that actin is mainly present as a submembraneous layer. The actin concentration detectable in the cytoplasm was about 16X lower. F-actin staining was increased mainly at the contracted tail and to a lesser extent in lamellipodia. However, there is also accumulation of the cell surface membrane at these sites. The quantitative analysis of electron micrographs showed that the apparent accumulation of F-actin at the tail and in the leading lamellipodia was, on the average, fully explained by increased membrane folding. The cell membrane as well as the cortical actin may fold and unfold during shape changes and polarized cells have reserves of plasma membrane as well as of cortical actin at the tail. In addition, the cells may show spots where the surface membrane was dissociated from the cortical actin layer. Polarized cells showed no increase in actin within the blebs or at the basis of lamellipodia. In this respect, the distribution of polymerized actin was different from other currently studied locomoting metazoan cells. So far, the data are difficult to reconcile with models, postulating that polymerized actin within the protrusions is the direct force driving the membrane forward.