TARGETING OF LISTERIA-MONOCYTOGENES ACTA PROTEIN TO THE PLASMA-MEMBRANE AS A TOOL TO DISSECT BOTH ACTIN-BASED CELL MORPHOGENESIS AND ACTA FUNCTION

Actin assembly an the surface of Listeria monocytogenes in the cytopla sm of infected cells provides a model to study actin-based motility an d changes in cell shape, We have shown previously that the ActA protei n, exposed on the bacterial surface, is required for polarized nucleat ion of actin filaments, To investigate whether plasma membrane-associa ted ActA can control the organization of microfilaments and cell shape , variants of ActA, in which the bacterial membrane signal had been re placed by a plasma membrane anchor sequence, were produced in mammalia n cells, While both cytoplasmic and membrane-bound forms of ActA incre ased the F-actin content, only membrane-associated ActA caused the for mation of plasma membrane extensions, This finding suggests that ActA acts as an actin filament nucleator and shows that permanent associati on with the inner face of the plasma membrane is required for changes in cell shape, Based on the observation that the amino-terminal segmen t of ActA and the remaining portion which includes the proline-rich re peats cause distinct phenotypic modifications in transfected cells, we propose a model in which two functional domains of ActA cooperate in the nucleation and dynamic turnover of actin filaments, The present ap proach is a new model system to dissect the mechanism of action of Act A and to further investigate interactions of the plasma membrane and t he actin cytoskeleton during dynamic changes of cell shape.