Chemotactic motion of coronin-null cells with impaired actin turnover

Previous studies have shown that Dictyostelium discoideum cells lacking the actin-regulating protein coronin A have a large hyaline zone at the front of the cell. However, the coronin mutant cells can efficiently navigate in a gradient of chemoattractant by extending rounded protrusions from the hyaline zone. This study examines whether this zone is occupied by actin filaments, as was previously assumed, or if it is free of filamentous actin, as would be the case for typical blebs. The lack of coronin A results in a large hyaline region in the anterior part of mutant cells, from which the endoplasmic reticulum is displaced. This zone is populated with filamentous actin. Despite this broadened front, the coronin-null cells can respond in a gradient of chemoattractant even when they lack actin-based structures such as filopodia or lamellipodia. During re-orientation into a changed direction of the gradient, the mutant cells form rounded protrusions in various directions, of which the one pointing to the direction of the gradient will expand and become the new front. Contact with a substratum is not necessary for a protrusion to be formed in the right direction. These results illustrate the altered but efficient chemotactic responses of cells under conditions of diminished actin filament turnover. Despite an impaired actin filament turnover, Dictyostelium discoideum cells lacking coronin A respond to a gradient of chemoattractant by extending rounded protrusions from their large hyaline fronts. These protrusions are rich in filamentous actin, and filopodia or lamellipodia are not required. This mode of chemotactic migration is different from a bleb-driven mode described for Dictyostelium and other eukaryotic cells.