STAPHYLOCOCCAL PROTEIN-A SIMULTANEOUSLY INTERACTS WITH FRAMEWORK REGION 1, COMPLEMENTARITY-DETERMINING REGION 2, AND FRAMEWORK REGION 3 ON HUMAN V(H)3-ENCODED IGS

Staphylococcal protein A (SPA) is a B cell superantigen that binds to human V(H)3-encoded Igs independently of the D- and J(H)-encoded regio ns and light chain sequences. The SPA-binding structure formed by V(H) 3-encoded Igs remains controversial. We localized the regions in a V(H )3-encoded Ab required for SPA binding by producing mutant Abs in the baculovirus expression system in which regions of a human-derived Ab k nown to bind SPA were exchanged with those from a mouse Ab of the J558 family, a family not associated with SPA binding, The pattern of SPA binding indicates not only that residues in FR1, CDR2, and FR3 are inv olved but also that the three regions are required to interact simulta neously with SPA for binding to occur, When any one of the three regio ns was replaced with the corresponding region from the nonbinding Ab, SPA binding was severely disrupted, These data indicate that SPA requi res simultaneous interaction with three distinct regions of a V(H)3 st ructure, which together in three-dimensional space form an extended so lvent-exposed surface. These studies more precisely define the genetic requirements for V(H)3-encoded Ig binding to SPA.