Caveolin-1 Mediates Salmonella Invasion via the Regulation of SopE-dependent Rac1 Activation and Actin Reorganization

Abstract

Caveolar endocytosis has an important function in the cellular uptake of some bacterial toxins, viruses, and circulating proteins. However, the molecular machinery involved in caveolae-dependent bacterial endocytosis is poorly defined. In the present study, we identify a new molecular mechanism for the caveolin-1-dependent entry of Salmonella into host cells via the direct regulation of actin reorganization. In contrast to the interaction of caveolae with other pathogens, the caveolae did not form Salmonella-containing vesicles or endosomes in the host cells. Instead, the caveolae rapidly moved to the apical plasma membrane upon actin condensation during early invasion. Interestingly, the injected bacterial protein SopE interacted with Rac1 to regulate actin reorganization, and both proteins colocalized and directly interacted with caveolin-1 in caveolae during early invasion. After the complete internalization of Salmonella, SopE levels decreased both in the caveolae and in the host cytoplasm; Rac1 activity was also decreased. Downregulation of caveolin-1 by siRNA treatment led to reduction of Salmonella invasion compared with control siRNA-treated cells. These results suggest a new model in which caveolin-1 might be involved in Salmonella entry via its interaction with SopE and Rac1, leading to enhanced membrane ruffling for phagocytosis into host cells.