Identification and Characterization of a Smad2 Homologue from Schistosoma mansoni, a Transforming Growth Factor-β Signal Transducer

Smad proteins are essential intracellular signal transducers of the transforming growth factor-β (TGF-β) superfamily. The TGF-β superfamily signals through phosphorylation and activation of R-Smad proteins, receptor-regulated Smads, by heteromeric complexes of ligand-specific type I and type II serine/threonine kinase receptors. R-Smads receive a signal from the activated receptor complex and transmit it to the nucleus. A cDNA was isolated that encodes a 649-amino acid protein found to be homologous to members of R-Smad subfamily with highest homology scored to clawed African frog and human Smad2. The Schistosoma mansoni homologue (SmSmad2) was overexpressed in bacteria as a Sj26-GST fusion protein and used to raise specific antibodies. The IgG fraction of the immunized rabbit serum identified 70- and 72-kDa protein bands in Western analysis of schistosome extracts. Treatment with alkaline phosphatase removed the 72-kDa band, which indicates that this band represents the phosphorylated form of schistosome Smad2. SmSmad2 was localized in the subtegument, parenchymal cells, and sex organs in both male and female worm cryosections. Similar results were also obtained from the analysis of the Smad2 mRNA distribution pattern revealed by in situ hybridization of adult worm pair paraffin sections. SmSmad2 mRNA levels were determined by reverse transcriptase-polymerase chain reaction in different mammalian host developmental stages and found to be constitutively expressed. SmSmad2 was also found to interact with a previously identified SmTβR-I, a serine/threonine type I kinase receptor. Furthermore, SmSmad2 was shown to undergo phosphorylation by constitutively active forms of SmTβR-I in vitro. In addition, SmSmad2 localized in the nuclei of mink lung epithelial cells upon treatment with TGF-β₁. These data indicate that the SmSmad2 responds to the TGF-β signals by interaction with receptor I, which phosphorylates it, whereupon it translocates into the nucleus presumably to regulate target gene transcription and consequently elicit a specific TGF-β effect.