The actin-binding properties of the Physarum actin-fragmin complex. Regulation by calcium, phospholipids, and phosphorylation.
The actin-binding properties of the actin-fragmin complex from Physarum polycephalum microplasmodia were investigated with respect to regulation by Ca2+, phospholipids, and phosphorylation of the actin subunit by the endogenous actin-fragmin kinase. Fragmin possesses two high affinity actin-binding sites and probably also a third, low affinity site. Its nucleating and F-actin severing activities are inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2). Actin-fragmin specifically binds PIP2 which competes with actin for the Ca(2+)-sensitive site. However, PIP2 cannot dissociate the actin-fragmin complex nor the actin2-fragmin trimer. Efficient F-actin nucleating activity by actin-fragmin is only observed with unphosphorylated actin-fragmin, in the absence of PIP2 and at high Ca2+ (> microM) concentrations. In the presence of PIP2, actin-fragmin only caps actin filaments when unphosphorylated. The results suggest that in the cell, hydrolysis of PIP2, concomitant with the increase of cytosolic Ca2+, could promote subcortical actin polymerization.
Department of Biochemistry, Universiteit Gent, Belgium.
J
J
Gettemans
Y
Y
De Ville
E
E
Waelkens
J
J
Vandekerckhove
D000199:Actins / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D017985:Dalteparin / Q000378:metabolism*; D004533:Egtazic Acid / Q000378:metabolism; D008969:Molecular Sequence Data; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism*; D010766:Phosphorylation; D010804:Physarum / Q000378:metabolism*; D011485:Protein Binding
D000199:Actins; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates; D010743:Phospholipids; D004533:Egtazic Acid; D017985:Dalteparin; D002118:Calcium