The invasion of epithelial cells by N. gonorrheae is accompanied by formation of a halo of actin filaments around the enveloped bacterium. The transfer of the bacterial major outer membrane protein, porin, to the host cell membrane during invasion makes it a candidate for a facilitator for the formation of this halo. Western analysis shows here that gonococcal porin P.IB associates with the actin cytoskeleton in infected cells. Using the pyrene-labeled Mg forms of yeast and muscle actins, we demonstrate that under low ionic strength conditions, P.IB causes formation of filamentous actin assemblies, although they, unlike F-actin, cannot be internally cross-linked with N,N'-4-phenylenedimaleimide (PDM). In F-buffer, low porin concentrations appear to accelerate actin polymerization. Higher P.IB concentrations lead to the formation of highly decorated fragmented F-actin-like filaments in which the actin can be cross-linked by PDM. Co-assembly of P.IB with a pyrene-labeled mutant actin, S(265)C, prevents formation of a pyrene excimer present with labeled S(265)C F-actin alone. Addition of low concentrations of porin to preformed F-actin results in sparsely decorated F-actin. Higher P.IB concentrations extensively decorate the filaments, thereby altering their morphology to a state like that observed when the components are copolymerized. With preformed labeled S(265)C F-actin, P.IB quenches the pyrene excimer. This decrease is prevented by the F-actin stabilizers phalloidin and to a lesser extent beryllium fluoride. P.IB's association with the actin cytoskeleton and its ability to interact with and remodel actin filaments support a direct role for porin in altering the host cell cytoskeleton during invasion.