Formation of the hetero-tetrameric TGF-beta-1 receptor complex induces receptor rotation, so that TGFBR2 and TGFBR1 cytoplasmic kinase domains face each other in a catalytically favourable configuration. The constitutively active type II receptor kinase (which auto-phosphorylates in the absence of ligand), trans-phosphorylates specific serine residues at the conserved Gly-Ser-rich juxtapositioned domain (GS domain) of the type I receptor (Wrana et al. 1994, Souchelnytskyi et al. 1996).
In addition to phosphorylation, TGFBR1 may also be sumoylated in response to TGF-beta-1 stimulation. Sumoylation enhances TGFBR1 function by facilitating recruitment and phosphorylation of SMAD3 (Kang et al. 2008).