Bidirectional polymerization of G-actin on the human erythrocyte membrane.

Open Access
Publisher Website
Google Scholar
Abstract
The directional polymerization of actin on the erythrocyte membrane has been examined at various concentrations of G-actin by thin-section EM. A new experimental system using single-layered erythrocyte membranes with the cytoplasmic surfaces freely exposed was developed. The preformed actin filaments did not bind with the cytoplasmic surface of the erythrocyte membranes. When the erythrocyte membranes were incubated at low concentrations (0.3 and 0.5 .mu.M) of G-actin, > 80% of polymerized actin filaments pointed toward the membranes mainly in an end-on fashion, as judged by arrowhead formation with heavy meromysin. At higher concentrations (2 and 4 .mu.M) of G-actin, about half of the polymerized actin filaments were directed with arrowheads pointing toward the membranes, while the rest of the filaments showed the opposite polarity pointing away from the membranes. The majority of polymerized actin filaments formed loops at the points of attachment to the membranes. When G-actin (2 and 4 .mu.M) in the presence of cytochalasin B was polymerized into filaments, .apprx. 70% showed the polarity pointing away from the membrane mainly in an end-on fashion. To check the treadmilling phenomena, the erythrocyte membranes with bidirectionally polymerized actin filaments were further incubated with G-actin at the overall critical concentration. Almost all (90%) of actin filaments showed the polarity with arrowheads pointing toward the membranes. The results obtained are discussed with special reference to the mode of association of actin filaments with the plasma membrane in general.