Type III TGF-β receptor-independent signalling of TGF-β2 via TβRII-B, an alternatively spliced TGF-β type II receptor

Transforming growth factor-β (TGF-β) signals through membrane-bound serine/threonine kinase receptors, which upon stimulation phosphorylate Smad proteins and thereby trigger their nuclear translocation and transcriptional activity. Although the three mammalian isoforms of TGF-β are highly homologous at the level of sequence, analysis of their in vivo function by gene knockouts revealed striking differences, suggesting no significant functional redundancy between TGF-β1, -2 and -3. While signal transduction by TGF-β1 has been well characterized, receptor binding and activation by the TGF-β2 isoform is less well understood. Here, we show that TβRII-B, an alternatively spliced variant of the TGF-β type II receptor, is a TGF-β2 binding receptor, which mediates signalling via the Smad pathway in the absence of any TGF-β type III receptor (TβRIII). L6 cells lacking endogenous TβRIII as well as TβRII-B do not respond to TGF-β2. Transfection of these cells with TβRII-B restores TGF-β2 sensitivity. The expression of TβRII-B is restricted to cells originating from tissues such as bone where the isoform TGF-β2 has a predominant role. This reflects the importance of this receptor in TGF-β isoform-specific signalling.