Twenty-Eight-Kilodalton Phosphorylatable Calcium- and Lipid-Binding Proteins Purified fromPhysarumPlasmodium

摘要

Two 28-kDa calcium- and lipid-binding proteins were isolated from a detergent-insoluble fraction of thePhysarumplasmodium. Both proteins have molecular masses of approximately 28 kDa by SDS-PAGE. The protein designated 28K-I has a slightly lower mobility than that designated 28K-II. The purified 28K-I has a dissociation constant of 1.0μM for Ca2+ions, while the 28K-II has two different dissociation constants: one of 0.32μM and the other of 3.2 mM. The 28K-I binds to liposomes at Ca2+concentrations higher than 1.0μM and has a dissociation constant for lipid of 34μg/ml at 10μM Ca2+. The 28K-II binds to liposomes at concentrations of Ca2+above the mM range and has a dissociation constant of 36μg/ml for lipid at 2 mM Ca2+. There is no evidence of actin-binding activity by either of the 28-kDa (28K) proteins. The 28K proteins crossreacted with an antiserum against chicken brush border calpactin I. The two proteins have quite different phosphorylation levels between a fraction prepared from the cytosolic endoplasm and a fraction prepared from the whole cell. The 28K proteins may play some role in the membrane structure dynamics of the cortical gel