Staphylococcal Protein A and Human IgG Subclasses and Allotypes

Staphylocoecal protein A binds molecules belonging to the IgGl, IgG2, and IgG4 sub‐classes. IgG3 proteins generally do not bind, except for those coded by the two γ3 alleles, which are G3m(u−): G3m(b0,b3,b5,s,v) and G3m(b0,b3,b5,s,t,v). G3m(u) is located in the CH2 domain. The difference between G3m(u−) and G3m(u+) IgG3 proteins correlates with the sequence at position 339 in the CH2 domain—Ala and Thr respectively. There is another structural difference in the CH3 domain which correlates with protein A binding and non‐binding: all IgG proteins that bind protein A have His at position 435, whereas those that do not, have Arg at that position.