Smad and AML proteins synergistically confer transforming growth factor β1 responsiveness to human germ-line IgA genes

Transcription of germ-line immunoglobulin heavy chain genes conditions them to participate in isotype switch recombination, Transforming growth factor-beta 1 (TGF-beta 1) stimulates promoter elements located upstream of the IgA1 and IgA2 switch regions, designated I alpha 1 and I alpha 2, and contributes to the development of IgA responses. We demonstrate that intracellular Smad proteins mediate activation of the I alpha 1 promoter by TGF-beta. TGF-beta type 1 receptor (ALK-5), activin type IB receptor (ALK-4), and the "orphan" ALK-7 trans-activate the I alpha 1 promoter, thus raising the possibility that other members of the TGF-beta superfamily can also modulate IgA synthesis. Smads physically interact with the AML family of transcription factors and cooperate with them to activate the I alpha 1 promoter. The I alpha 1 element provides a canape of interspersed high and low affinity sites for Smad and AML factors, some of which are indispensable for TGF-beta responsiveness. While AML Smad complexes are formed in the cytoplasm of DG75 and K562 cells constitutively, only after TGF-beta receptor activation, novel Smad3.Smad4.AML complexes are detected in nuclear extracts by EMSA with I alpha 1 promoter-derived probes. Considering the wide range of biological phenomena that AMLs and Smads regulate, the physical/ functional interplay between them has implications that extend beyond the regulation of class switching to IgA.