Evidence for a direct, nucleotide-sensitive interaction between actin and liver cell membranes

We have investigated the association of actin with membranes isolated from rat liver. A plasma membrane-enriched fraction prepared by homogenization in a low salt/CaCl2 buffer was found to contain a substantial amount of residual actin which could be removed by treatment with 1 M Na2CO3/NaHCO3, pH 10.5. Using a sedimentation binding assay that uses gelsolin to shorten actin filaments and render membrane binding saturable (Schwartz, M. A., and E. J. Luna. 1986. J. Cell Biol. 102:2067-2075), we found that membranes stripped of endogenous actin bound 125I-actin in a specific and saturable manner. Scatchard plots of binding data were linear, indicating a single class of binding sites with a Kd of 1.6 microns; 66 micrograms actin bound/mg membrane protein at saturation. Binding of actin to liver cell membranes was negligible with unstripped membranes, was competed by excess unlabeled actin, and was greatly reduced by preheating or proteolytic digestion of the membranes. Kinetic measurements showed that binding had an initial lag phase and was strongly temperature dependent. The binding of actin to liver cell membranes was also found to be competitively inhibited by ATP and other nucleotides, including the nonhydrolyzable analogue AMP-PNP. We conclude that we have reconstituted an interaction between actin and integral membrane proteins from the rat liver. This interaction exhibits a number of distinctive features which have not been observed in other actin- membrane systems.