Identification ofTwoFunctional FormsofImmunoglobulin G3-Binding Protein Expressed byGroupA Streptococci

Analysis ofgroupA streptococcal immunoglobulin G (IgG)-binding protein reactivity withdifferent human IgG3-myeloma proteins provided evidence foratleast twofunctional formsofthese molecules. Representative IgG3-binding molecules wereisolated, biotinylated, andusedastracers incompetitive binding assays. Cross-inhibition studies demonstrated theexistence oftwodistinct patterns ofIgG3-binding activity. Proteins ofoneformcouldbeinhibited frombinding toanIgG3-myeloma protein bystreptococcal protein G while binding ofthesecond formwasnotinhibited. Thesestudies further underscore theextent ofheterogeneity amongimmunoglobulin-binding proteins expressed bygroupA streptococci. TypeIIimmunoglobulin G (IgG)-binding proteins ex- pressed bygroupA streptococci represent themostdiverse groupofbacterial IgG-binding proteins identified todate (Tablel). Fivefunctionally distinct formsoftypeIIIgG- binding proteins that differ intheir nonimmune reactivity with humanIgGsubclasses andIgGimmunoglobulins fromother mammalian species havebeencharacterized (8). Recent stud- iesfromourlaboratory havedemonstrated thatthevast majority ofIgG-binding proteins extracted fromgroupA streptococci canbedivided into twomajor families onthebasis ofrecognition byapanel ofpolyclonal monospecific chicken antibodies (9). Polyclonal antibodies toatypeIIoIgG-binding protein isolated fromstrain A928reacted withthemajority of IgG-binding proteins expressed byopacity-factor-negative iso- lates (2, 9). Antibodies totheproduct ofthefcrA gene(pLOH) ofstrain 64/14reacted withthemajority ofisolated IgG- binding proteins fromopacity-factor-positive streptococci (2, 9). Multiple functionally distinct forms ofIgG-binding proteins wererecognized bythese antibody probes (9). Functional andserological analyses oftheimmunoglobulin- binding proteins expressed bygroupA streptococcal isolate 64/14 revealed thatthiswasanunusual strain inthatit expressed three distinct IgG-binding molecules identified in CNBrextracts (7). Thefirst, an -45,000-Mr protein, was reactive withhumanIgGl, IgG2, andIgG4(type Ila) andwas recognized bytheanti-Ilo antibody probe(2). Thesecond protein, an-33,000-M, doublet typeIlaprotein, wasantigeni- cally distinct andwasrecognized bytheanti-pLOH antibody probe(2,7).A third protein, an-34,000-M, IgG3-binding protein (type Ilb), wasalso present inCNBrextracts ofstrain 64/14, butthis immunoglobulin-binding