THE CELL CORTEX OF DICTYOSTELIUM DISCOIDEUM: A DYNAMIC ACTIN-CONTAINING COMPARTMENT (BIOLOGY, MOTILITY, CYTOSKELETON)

Abstract

The cell cortex in vegetative amoebae of Dictyostelium discoideum is an actin-containing compartment which is subjacent to the plasma membrane and surrounds the entire cell. The cortex is enriched in actin, myosin, and two actin-binding proteins, 95K and 120K. Actin filaments, which are enriched in the cortex, are either directly or indirectly attached to the cell membrane. Evidence from a number of laboratories has demonstrated that the cortical actin-containing cytoskeleton is responsible for facilitating cell movement and shape changes.;I have isolated the cortex from vegetative amoebae of Dictyostelium discoidenum by sucrose density centrifugation and have examined it both ultrastructurally and biochemically. I have shown that the structure and composition of the cortex can be altered by isolating it under certain conditions. A change in pH and/or pCa, or the addition of Con A induces dramatic results both at the electron microscope and biochemical level.;Actin and myosin are recruited to the membrane when Con A is bound to the extracellular surface. I have investigated the mechanism of assembly of both actin and myosin in the cortex during ligand binding by the DNase I assay and by in vivo phosphorylation, respectively. Recruitment of actin does not involve polymerization, while dephosphorylation of myosin heavy chain suggests that polymerization of myosin thick filaments is the mechanism for myosin recruitment.;The addition of the ligand, Concanavalin A, to intact Dictyostelium amoebae, results in the movement of ligand receptor complexes on the cell surface, in a process called capping. The actin-containing cytoskeleton found in the cell cortex is thought to be responsible for this movement. In an effort to further elucidate the roles played by actin, myosin, 95K and 120K during ligand binding, I have isolated and raised antibodies against these four antigens. The antibodies were affinity purified on antigen-linked Sepharose 4B columns and their monospecificity against the proteins found in D.d. whole cell homogenates and isolated cortices was confirmed by western blot analysis. I have used the affinity purified antibodies to follow the location and redistribution of the cortical proteins by indirect immunofluorescence during ligand binding.;The cortical proteins have distinct but different locations in the cell both before and after ligand challenge. Actin, myosin and 95K proteins all co-patch and co-cap with ligand. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.