Genetic dissection of the Streptococcus pyogenes M1 protein: regions involved in fibronectin binding and intracellular invasion

Abstract

Entry of serotype M1 Streptococcus pyogenes into host cells depends on binding of the host glycoprotein fibronectin (Fn) by the bacterial M1 protein. The present study was undertaken to localize the Fn binding region in M1 and assess other potential functions of M1. A set of recombinant M1 protein fragments were assayed for their capacities to bind Fn and inhibit ingestion of streptococci by epithelial cells. M1 protein, M6 protein and internally-deleted derivatives of M1 were expressed on the surface of Lactococcus lactis. Lactococci that expressed M1 or M6 protein bound Fn and were efficiently taken up by epithelial cells. Deletion of both the N-terminal A and B repeats regions of M1 abrogated Fn binding and intracellular invasion. Deletion of either the A domain (M1ΔA) or B repeats (M1ΔB) significantly reduced, but did not completely eliminate, Fn binding indicating that M1 protein may possess two independent Fn binding sites. Fn binding by the M1ΔA or M1ΔB proteins was insufficient for efficient invasion, however, suggesting that M protein binding alters the structure of Fn that, in turn, affects the interaction between Fn and epithelial cells. Although expression of M1, M6 or M1ΔB proteins led to aggregation of lactococcal cells, aggregation did not significantly contribute to invasion efficiency.