Motility of ActA protein-coated microspheres driven by actin polymerization

Actin polymerization is required for the generation of motile force at the leading edge of both lamellipodia and filopodia and also at the surface of motile intracellular bacterial pathogens such as Listeria monocytogenes, Lo cal catalysis of actin filament polymerization is accomplished in L, monocy togenes by the bacterial protein ActA, Polystyrene beads coated with purifi ed ActA protein can undergo directional movement in an actin-rich cytoplasm ic extract. Thus, the actin polymerization based motility generated by ActA can be used to move nonbiological cargo, as has been demonstrated for clas sical motor molecules such as kinesin and myosin, Initiation of unidirectio nal movement of a symmetrically coated particle is a function of bead size and surface protein density. Small beads (less than or equal to 0.5 mu m in diameter) initiate actin-based motility when local asymmetries are built u p by random fluctuations of actin filament density or by thermal motion, de monstrating the inherent ability of the dynamic actin cytoskeleton to spont aneously self-organize into a polar structure capable of generating unidire ctional force. Larger beads (up to 2 mu m in diameter) can initiate movemen t only if surface asymmetry is introduced by coating the beads on one hemis phere, This explains why the relatively large L, monocytogenes requires pol ar distribution of ActA on its surface to move.