F-actin capping by cap32/34 requires heterodimeric conformation and can be inhibited with PIP2

The heterodimeric F-actin capping protein cap32/34 from Dictyostelium discoideum is a typical member of a widely distributed family of cytoskeletal proteins. To analyze its regulation and structure/function relationships we cloned and expressed the subunits separately in Escherichia coli using the ATG-expression vector pT7-7. Studies on the viscosity of F-actin solutions and the kinetics of actin polymerization in the presence of single subunits or the reconstituted protein showed that capping of F-actin absolutely requires the heterodimeric conformation. This activity can be inhibited by phosphatidyl bisphosphate (PIP2), an important component in signal transduction. The regulation of cap32/34 by PIP2 suggests an involvement of this protein in the re-organization of the actin cytoskeleton upon stimulation of D. discoideum cells with chemoattractant.