CORTEXILLINS, MAJOR DETERMINANTS OF CELL-SHAPE AND SIZE, ARE ACTIN-BUNDLING PROTEINS WITH A PARALLEL COILED-COIL TAIL

Cortexillins I and II of D. discoideum constitute a novel subfamily of proteins with actin-binding sites of the alpha-actinin/spectrin type. The C-terminal halves of these dimeric proteins contain a heptad repe at domain by which the two subunits are joined to form a two-stranded, parallel coiled coil, giving rise to a 19 nm tail. The N-terminal dom ains that encompass a consensus actin-binding sequence are folded into globular heads. Cortexillin-linked actin filaments form preferentiall y anti-parallel bundles that associate into meshworks. Both cortexilli ns are enriched in the cortex of locomoting cells, primarily at the an terior and posterior ends. Elimination of the two isoforms by gene dis ruption gives rise to large, flattened cells with rugged boundaries, p ortions of which are often connected by thin cytoplasmic bridges. The double-mutant cells are multinucleate owing to a severe impairment of cytokinesis.