POLARIZED DISTRIBUTION OF LISTERIA-MONOCYTOGENES SURFACE PROTEIN ACTAAT THE SITE OF DIRECTIONAL ACTIN ASSEMBLY

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