Polarized distribution of Listeria monocytogenes surface protein ActA at the site of directional actin assembly

The facultative intracellular pathogen Listeria monocytogenes can infect host tissues by using directional actin assembly to propel itself from one cell into another. The movement is generated by continuous actin assembly from one end of the bacterium into a tail, which is left behind in the cytoplasm. Bacterial actin assembly requires expression of the bacterial gene actA. We have used immunocytochemistry to show that the actA gene product, ActA, is distributed asymmetrically on the bacterial surface: it is not expressed at one pole and is increasingly concentrated towards the other. This polarized distribution of ActA was linked to bacterial division: ActA protein was not, or only faintly, expressed at the pole that had been formed during the previous division. On intracellular bacteria ActA was expressed at the site of actin assembly, suggesting that ActA may be involved in actin filament nucleation off the bacterial surface. We predict that the asymmetrical distribution of this protein is required for the ability of intracellular Listeria to move in the direction of the non-ActA expressing pole.