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Tfg (Trk fused gene) is a Carma-1/IKKγ interacting protein involved in CD40-induced canonical NF-κB signaling

Carma-1 is required for B cell receptor-/CD40- and T cell receptor-/CD28-induced B- and T-cell activation via JNK and NF-βB. In B cells, Carma-1 becomes phosphorylated by PKCβ, leading to its oligomerization. Subsequent Bcl10 binding induces IKKβ-activation and, thereby, canonical NF-κB signalling. Despite these findings it is still unknown how exactly Carma-1 is connected to the plasma membrane and to the IKK-complex. Therefore, we purified Carma-1 complexes from mouse CH12 B cells using anti-Carma-1 affinity columns. Mass spectrometric analyses of the column eluates demonstrated the presence of Carma-1 as well as three previously uncharacterized adaptor proteins in B cells, one of which was the Trk-fused gene (Tfg), an adaptor protein containing PB1 and coiled-coil domains. Whereas Tfg was originally identified as fusion partner of oncogenic Trk tyrosine kinase mutants, the normal cellular homologue of Tfg has so far not been described in B cells. However, Tfg has been shown in other systems to interact with IKKγ and to enhance TNF-induced NF-κB activation.

Tfg and Carma-1 co-localized at the plasma membrane and perinuclear structures in B cells. We further corroborated the interactions of Tfg, IKKγ and Carma-1 by Blue Native gel electrophoresis, where Carma-1 and Tfg formed a 0.7–1 MDa complex. Ectopic expression of Tfg increased the molecular mass of IKKγ complexes, fused IKKγ, Bcl10 and Carma-1 complexes to a ~2 MDa complex, and increased basal and CD40-induced canonical activity of NF-κB and IKKβ. In contrast, shRNA-mediated silencing of Tfg decreased CD40-induced IKKβ activity.

Very interestingly, in primary B cells, highest expression of Tfg was detected in marginal zone and B1 B cells, and Carma-1 and Tfg formed complexes in these B cells. Since Carma-1 is required for marginal zone B cell and B1 B cell development, we suggest that a functional interaction between Carma-1 and Tfg contributes to development and maintenance of these cells by means of canonical NF-κB signals.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 2.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Grohmann, M., Hermann, I., Hampel, M. et al. Tfg (Trk fused gene) is a Carma-1/IKKγ interacting protein involved in CD40-induced canonical NF-κB signaling. Cell Commun Signal 7 (Suppl 1), A4 (2009).

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