Purification and characterization of a new mammalian serum protein with the ability to inhibit actin polymerization and promote depolymerization of actin filaments

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Abstract
A protein with capacity to bind G-actin and the ability to inhibit polymerization and promote depolymerization of actin filaments was isolated from the serum of rabbit. The protein, SAIP (for serum actin inhibitory protein), was purified by affinity chromatography of serum over actin-Sepharose followed by protein fractionation with ammonium sulfate and chromatography over DEAE-cellulose. Five milligrams of purified SAIP is obtained from 100 ml of serum. Rabbit SAIP is resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis into 2 closely related polypeptides of 60,000 and 56,000 daltons, respectively (ratio 5.7:1). Each of these polypeptides consists of 2 isoelectric variants. SAIP binds to monomeric actin with a stoichiometry of 1:1 and a Kd of 0.12 .mu.M. The SAIP-actin complex binds to DNase I. Actin polymerization is completely inhibited by incubation of actin with an equal concentration of SAIP. At equimolar concentrations to F-actin, SAIP induces complete depolymerization of the actin filaments. SAIP is also present in calf serum.