DECORATION WITH MYOSIN SUBFRAGMENT-1 DISRUPTS CONTACTS BETWEEN MICROFILAMENTS AND THE CELL-MEMBRANE IN ISOLATED DICTYOSTELIUM CORTICES

Isolated cortices from amoeboid cells of D. discoideum were used to examine the structural nature of attachments between microfilaments and the cell membrane and to determine the effect of myosin subfragment-1 (S-1) on such contacts. Two types of contact sites were identified between microfilaments and the cell membrane similar to those seen in the brush border of intestinal epithelial cells: an end-on attachment between the barbed end of actin filaments and the cell membrane; and a lateral attachment mediated by rod-shaped bridges measuring .apprx. 6 .times. 15 nm. The spacing between bridges averaged 36 nm, suggesting that the helical twist of the actin filament influences bridge location. Together these contacts accounted for an average of .apprx. 25,000 attachments/cell. Incubation of cortices with concentrations of S-1 sufficient to saturate binding sites on the microfilaments caused disruption of the contacts. This observation was confirmed by quantitative morphometry to show a 3-fold loss in the number of contact sites following S-1 decoration. S-1 decoration should be used with caution when information about the precise location of microfilaments and their attachment to the membrane is required.