ASSOCIATION OF THE DICTYOSTELIUM 30 KDA ACTIN BUNDLING PROTEIN WITH CONTACT REGIONS

'Contact regions' are plasma membrane domains derived from areas of in tercellular contact between aggregating Dictyostelium amebae (H. M. In galls et al. (1986). Proc. Nat. Acad. Sci. USA 83, 4779). Purified con tact regions contain a prominent actin-binding protein with an M(r) of 34,000. Immunoblotting with monoclonal antibodies identifies this pol ypeptide as a 34,000 M(r) actin-bundling protein (known as 30 kDa prot ein), previously shown to be enriched in filopodia (M. Fechheimer (198 7). J. Cell Biol. 104, 1539). About four times more 30 kDa protein by mass is associated with contact regions than is found in total plasma membranes isolated from aggregating cells. In agreement with these obs ervations, immunostaining of the 30 kDa protein in aggregating cells r eveals a prominent localization along the plasma membrane at sites of intercellular contact. By contrast, alpha-actinin does not appear to b e significantly enriched at sites of cell to cell contact. Binding exp eriments using purified plasma membranes, actin and 30 kDa protein ind icate that the 30 kDa protein is associated with the plasma membrane p rimarily through interactions with actin filaments. Calcium ions are k nown to decrease the interaction of actin with 30 kDa protein in solut ion. Surprisingly, membrane-associated complexes of actin and the 30 k Da protein are much less sensitive to dissociation by micromolar level s of free calcium ions than are complexes in solutions lacking membran es. These results suggest that the interaction of the 30 kDa protein w ith F-actin at regions of cell-cell contact may be less sensitive to d isruption by free calcium ions than elsewhere in the cell cortex. The positively cooperative assembly of stable complexes of actin and the 3 0 kDa protein at contact regions may be an important factor in the org anization of both the cortex and these membrane domains that are speci alized for intercellular adhesion.