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The Scl1 protein of M6‐type group A Streptococcus binds the human complement regulatory protein, factor H, and inhibits the alternative pathway of complement
Author(s) -
Caswell Clayton C.,
Han Runlin,
Hovis Kelley M.,
Ciborowski Pawel,
Keene Douglas R.,
Marconi Richard T.,
Lukomski Slawomir
Publication year - 2008
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/j.1365-2958.2007.06067.x
Subject(s) - antibody opsonization , factor h , biology , complement system , complement factor i , phagocytosis , affinity chromatography , protein g , glycoprotein , binding protein , plasma protein binding , complement control protein , alternative complement pathway , molecular mass , biochemistry , recombinant dna , microbiology and biotechnology , antibody , opsonin , enzyme , gene , immunology
Summary Non‐specific activation of the complement system is regulated by the plasma glycoprotein factor H (FH). Bacteria can avoid complement‐mediated opsonization and phagocytosis through acquiring FH to the cell surface. Here, we characterize an interaction between the streptococcal collagen‐like protein Scl1.6 of M6‐type group A Streptococcus (GAS) and FH. Using affinity chromatography with immobilized recombinant Scl1.6 protein, we co‐eluted human plasma proteins with molecular weight of 155 kDa, 43 kDa and 38 kDa. Mass spectrometry identified the 155 kDa band as FH and two other bands as isoforms of the FH‐related protein‐1. The identities of all three bands were confirmed by Western immunoblotting with specific antibodies. Structure–function relation studies determined that the globular domain of the Scl1.6 variant specifically binds FH while fused to collagenous tails of various lengths. This binding is not restricted to Scl1.6 as the phylogenetically linked Scl1.55 variant also binds FH. Functional analyses demonstrated the cofactor activity of the rScl1.6‐bound FH for factor I‐mediated cleavage of C3b. Finally, purified FH bound to the Scl1.6 protein present in the cell wall material obtained from M6‐type GAS. In conclusion, we have identified a functional interaction between Scl1 and plasma FH, which may contribute to GAS evasion of complement‐mediated opsonization and phagocytosis.