Roles of integrins and fibronectin in the entry of Streptococcus pyogenes into cells via protein F1

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Abstract
Entry of group A streptococcus (GAS) into cells has been suggested as an important trait in GAS pathogenicity. Protein F1, a fibronectin (Fn) binding protein, mediates GAS adherence to cells and the extracellular matrix, and efficient cell internalization. We demonstrate that the cellular receptors responsible for protein F1‐mediated internalization of GAS are integrins capable of Fn binding. In HeLa cells, bacterial entry is blocked by anti‐β1 integrin monoclonal antibody. In the mouse cell line GD25, a β1 null mutant, the αvβ3 integrin promotes GAS entry. Internalization of these cells by GAS is blocked by a peptide that specifically binds to αvβ3 integrin. In both cell lines, entry of GAS requires the occupancy of protein F1 by Fn. Neither the 29 kDa nor the 70 kDa N‐terminal fragments or the 120 kDa cell‐binding fragment of Fn promote bacterial entry. Fn‐coated beads are taken up efficiently by HeLa cells. Both the entry of GAS via protein F1 and the uptake of Fn‐coated beads are blocked by anti‐β1 antibody but are unaffected by a large excess of soluble Fn. Internalization of HeLa cells by bacteria bearing increasing amounts of prebound Fn to protein F1 reveals a sigmoidal ultrasensitive curve. These suggest that the ability of particles to interact via Fn with multiple integrin sites plays a central role in their ability to enter cells.