THE ACTA PROTEIN OF LISTERIA-MONOCYTOGENES ACTS AS A NUCLEATOR INDUCING REORGANIZATION OF THE ACTIN CYTOSKELETON

Listeria monocytogenes, a facultative intracellular pathogen, employs actin and other microfilament-associated proteins to move through the host cell cytoplasm. Isogenic mutants of L.monocytogenes lacking the s urface-bound ActA polypeptide no longer interact with cytoskeletal ele ments and are, as a consequence, non-motile (Domann et al., 1992, EMBO J., 11, 1981-1990; Rocks et al., 1992, Cell, 68, 521-531). To investi gate the interaction of ActA with the microfilament system in the abse nce of other bacterial factors, the listerial actA gene was expressed in eukaryotic cells. Immunofluorescence studies revealed that the comp lete ActA, including its C-terminally located bacterial membrane ancho r, colocalized with mitochondria in transfected cells. When targeted t o mitochondria, the ActA polypeptide recruited actin and alpha-actinin to these cellular organelles with concomitant reorganization of the m icrofilament system. Removal of the internal proline-rich repeat regio n of ActA completely abrogated interaction with cytoskeletal component s. Our results identify the ActA polypeptide as a nucleator of the act in cytoskeleton and provide the first insights into the molecular natu re of such controlling elements in microfilament organization.