Actin-associated proteins in motility and chemotaxis of Dictyostelium cells.

The amoeboid cells of Dictyostelium discoideum are amenable to a combined biochemical, genetic, and cell biological approach that can be focussed to the study of molecular interactions underlying the chemotactic responses of eukaryotic cells. In these responses the actin-based motility system is involved. This system is characterised in Dictyostelium cells by a large number and variety of regulatory proteins. Most of these proteins belong to families that are likewise represented in the cytoskeletons of higher eukaryotes including man. Elimination of some of these actin-binding proteins by chemical mutagenesis or gene disruption is being used to simplify the system by separating essential proteins from non-essential ones. These studies are complemented by the selection and analysis of mutants with altered motility or chemotaxis. Quantitative motion analysis of mutants is employed to establish a link between defects on the molecular level and alterations in cell behaviour. Dictyostelium cells respond to local stimulation by extending a newly formed leading edge towards a chemoattractant within less than a minute, thereby changing their polarity. The leading edge is formed by the recruitment of soluble proteins from the cytoplasm and their coassembly with actin into a complicated framework of microfilaments. Patterns of assembly are shown in this report for two proteins, the talin-like filopodin and coronin. Elucidation of the control mechanisms of this ordered assembly will provide the key for understanding the molecular processes responsible for a chemotactic response.