Regulation by Ca(2+)-calmodulin of the actin-bundling activity of Physarum 210-kDa protein.
From the plasmodia of a lower eukaryote, Physarum polycephalum, we have previously purified a 210-kDa protein that showed similar properties to those of smooth muscle caldesmon. Further characterization of the 210-kDa protein revealed that it bundled actin filaments. This bundling activity was inhibited by calmodulin in the presence of Ca2+. Unlike smooth muscle caldesmon, the 210-kDa protein bundled actin filaments whether or not a reducing agent, such as dithiothreitol, was present. The protein was shown to have two (or more) different actin-binding sites which were classified into salt-sensitive and salt-insensitive sites. Electron microscopy revealed that the 210-kDa protein was an elongated molecule (mean length, 97 +/- 25 nm) which was bent in the middle. The Stokes radius and sedimentation coefficient of the 210-kDa protein were 130 A and 2.9 S, respectively. An immunofluorescence study revealed that the 210-kDa protein colocalized with the bundles of actin filaments in thin-spread preparations of Physarum plasmodia, suggesting that the 210-kDa protein was regulating the appearance and disappearance of the actin bundles that are associated with the contraction-relaxation cycle of the plasmodia.
Ishikawa
R
R
Department of Pharmacology, Gunma University School of Medicine, Japan.
Okagaki
T
T
Kohama
K
K
D000199:Actins / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002147:Calmodulin / Q000502:physiology*; D002148:Calmodulin-Binding Proteins / Q000378:metabolism; D005656:Fungal Proteins / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D000070559:Gizzard, Non-avian; D008840:Microfilament Proteins / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D008854:Microscopy, Electron; D010804:Physarum / Q000378:metabolism*
D000199:Actins; D002147:Calmodulin; D002148:Calmodulin-Binding Proteins; D005656:Fungal Proteins; D008840:Microfilament Proteins; D002118:Calcium