Studies on microplasmodia of Physarum polycephalum: VIII. Visualization of a membrane skeleton and characterization of membrane proteins

Small plasmodial fragments derived from the acellular slime mold Physarum polycephalum were used to demonstrate a functional relationship between ultrastructural changes of the membrane-cytoskeleton complex and rhythmic activities of the motive force generating system. During contraction the microfilament layer detaches from the plasma membrane and moves inwards thus causing syneresis of central granuloplasm from peripheral hyaloplasm. Simultaneously, on the internal face of the plasma membrane a more or less regular two-dimensional network of short filaments and small particles becomes visible resembling the spectrin membrane skeleton of erythrocytes. With incipient relaxation the contracted microfilament system disintegrates and a new layer is formed at the internal cell surface. Biochemical studies on isolated plasma membranes by gel-electrophoresis revealed three distinct bands of 29, 38 and 40 kD as well as some other conspicuous polypeptides such as actin, myosin and, especially, a high molecular weight actin-binding protein (HABP) of 230 kD. Moreover, a 22 kD component was recognized as an integral membrane glycoprotein by Triton X-114 extraction and Con A-peroxidase staining. It is discussed whether the 230 kD and 22 kD proteins are comparable to spectrin and the actin-binding protein ponticulin from Dictyostelium, respectively.