A calcium- and pH-regulated protein from Dictyostelium discoideum that cross-links actin filaments.

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Abstract
An actin binding protein was purified from amebas of D. discoideum which was called 95,000-dalton protein (95K). This protein is rod-shaped, is .apprx. 40 nm long in the EM, contains 2 subunits 95,000 daltons each, and cross-links actin filaments. Cross-linking activity was demonstrated by using falling-ball viscometry, Ostwald viscometry, and EM. Cross-linking activity is optimal at 0.1 .mu.M Ca2+ and pH 6.8, but is progressively inhibited at higher Ca2+ and pH levels over a physiological range. Half-maximal inhibition occurs at 1.6 .mu.M free Ca2+ and pH 7.3, respectively. Sedimentation experiments demonstrate that elevated Ca2+ and pH inhibit the binding of 95K to F-actin which explains the loss of cross-linking activity. EM demonstrates that, under optimal conditions for cross-linking, 95K protein bundles actin filaments and that this bundling is inhibited by .mu.M Ca+. Severing of actin filaments by 95K was not observed in any of the various assays under any of the solution conditions used. Hence, 95K protein is a rod-shaped, dimeric, Ca2+- and pH-regulated actin binding protein that cross-links but does not sever actin filaments.