Binding of the Human Complement Regulators CFHR1 and Factor H by Streptococcal Collagen-like Protein 1 (Scl1) via Their Conserved C Termini Allows Control of the Complement Cascade at Multiple Levels

Michael Reuter,Clayton C Caswell,Slawomir Lukomski,Peter F Zipfel

doi:10.1074/jbc.m110.143727

Abstract

Group A streptococci (GAS) utilize soluble human complement regulators to evade host complement attack. Here, we characterized the binding of the terminal complement complex inhibitor complement Factor H-related protein 1 (CFHR1) and of the C3 convertase regulator Factor H to the streptococcal collagen-like proteins (Scl). CFHR1 and Factor H, but no other member of the Factor H protein family (CFHR2, CFHR3, or CFHR4A), bound to the two streptococcal proteins Scl1.6 and Scl1.55, which are expressed by GAS serotypes M6 and M55. The two human regulators bound to the Scl1 proteins via their conserved C-terminal attachment region, i.e. CFHR1 short consensus repeats 3-5 (SCR3-5) and Factor H SCR18-20. Binding was affected by ionic strength and by heparin. CFHR1 and the C-terminal attachment region of Factor H did not bind to Scl1.1 and Scl2.28 proteins but did bind to intact M1-type and M28-type GAS, which express Scl1.1 and Scl2.28, respectively, thus arguing for the presence of an additional binding mechanism to CFHR1 and Factor H. Furthermore mutations within the C-terminal heparin-binding region and Factor H mutations that are associated with the acute renal disease atypical hemolytic uremic syndrome blocked the interaction with the two streptococcal proteins. Binding of CFHR1 affected the complement regulatory functions of Factor H on the level of the C3 convertase. Apparently, streptococci utilize two types of complement regulator-acquiring surface proteins; type A proteins, as represented by Scl1.6 and Scl1.55, bind to CFHR1 and Factor H via their conserved C-terminal region and do not bind the Factor H-like protein 1 (FHL-1). On the contrary, type B proteins, represented by M-, M-like, and the fibronectin-binding protein Fba proteins, bind Factor H and FHL-1 via domain SCR7 and do not bind CFHR1. In conclusion, binding of CFHR1 is at the expense of Factor H-mediated regulatory function at the level of C3 convertase and at the gain of a regulator that controls complement at the level of the C5 convertase and formation of the terminal complement complex.

Highlights

Streptococcus pyogenes is a specialized Gram-positive ␤-hemolytic human pathogen
complement Factor H-related protein 1 (CFHR1) and Factor H Binding to streptococcal collagen-like proteins (Scl)—To characterize the role of Scl proteins for immune and complement escape of Group A streptococci (GAS), we analyzed the binding of recombinant Scl1.6 and Scl1.55 to four immobilized CFHR proteins (i.e. CFHR1, CFHR2, CFHR3, and CFHR4A), as well as to Factor H and Factor H-like protein 1 (FHL-1)
Streptococcal Scl2.28 was used as a negative control, and this protein did not bind to any tested Factor H family protein

Summary

Introduction

Streptococcus pyogenes is a specialized Gram-positive ␤-hemolytic human pathogen. Group A streptococci (GAS)[2] infections cause a variety of diseases including superficial infections of the throat and skin, such as pharyngitis and impetigo, and deep soft tissue infections like necrotizing fasciitis and myositis, as well as live threatening septic infections including toxic shock syndrome and puerperal sepsis (1, 2). Factor H-like protein 1 (FHL-1), which is derived from an alternatively spliced transcript of the Factor H gene, consists of the first seven SCRs including the regulatory region Both regulators act on the level of the C3 convertase, accelerating the decay of the convertase and acting as cofactors for the C3b-cleaving serine protease Factor I (13– 15). S. pyogenes expresses several surface proteins that recruit the host complement regulators Factor H, FHL-1, CFHR1, and C4-binding protein (C4BP) to the bacterial surface. This allows the pathogen to control host complement attack, to inactivate complement effector proteins, and to block C3b opsonization and the generation of inflammatory anaphylatoxins (C3a and C5a), as well as TCC formation and surface deposition. More than 140 GAS M-protein variants are identified on the basis of emm gene typing (19, 21–27)

Objectives

Results

Conclusion