A membrane cytoskeleton from Dictyostelium discoideum. I. Identification and partial characterization of an actin-binding activity

Elizabeth J. Luna, University of Massachusetts Medical School
V. M. Fowler
J. Swanson
D. Branton
D. L. Taylor

Abstract

Dictyostelium discoideum plasma membranes isolated by each of three procedures bind F-actin. The interactions between these membranes and actin are examined by a novel application of falling ball viscometry. Treating the membranes as multivalent actin-binding particles analogous to divalent actin-gelation factors, we observe large increases in viscosity (actin cross-linking) when membranes of depleted actin and myosin are incubated with rabbit skeletal muscle F-actin. Pre-extraction of peripheral membrane proteins with chaotropes or the inclusion of Triton X-100 during the assay does not appreciably diminish this actin cross-linking activity. Lipid vesicles, heat-denatured membranes, proteolyzed membranes, or membranes containing endogenous actin show minimal actin cross-linking activity. Heat-denatured, but not proteolyzed, membranes regain activity when assayed in the presence of Triton X-100. Thus, integral membrane proteins appear to be responsible for some or all of the actin cross-linking activity of D. discoideum membranes. In the absence of MgATP, Triton X-100 extraction of isolated D. discoideum membranes results in a Triton-insoluble residue composed of actin, myosin, and associated membrane proteins. The inclusion of MgATP before and during Triton extraction greatly diminishes the amount of protein in the Triton-insoluble residue without appreciably altering its composition. Our results suggest the existence of a protein complex stabilized by actin and/or myosin (membrane cytoskeleton) associated with the D. discoideum plasma membrane.