Tax neither interacts with Smads nor inhibits receptor-dependent formation of heteromers containing Smads.

(A) Tax does not interact directly with Smad proteins. Tax was transfected into COS7 cells with the indicated Flag-tagged or HA-tagged Smad constructs. Cell extracts were subjected to immunoprecipitation (IP) with anti-Flag and anti-HA. This was followed by immunoblotting with anti-Tax antibody. As positive control, whole-cell lysates from HTLV-I–infected HUT-102 cells were blotted directly with anti-Tax (lane labeled as HUT-102 cellular extract) (top). Immunoprecipitates were blotted with anti-Flag and anti-HA to control for Smad expression (middle). Cell lysates were blotted with anti-Tax to control for Tax expression (bottom). (B) Tax does not inhibit receptor-dependent formation of heteromers containing Smad2 and Smad4. COS7 cells were transfected with the indicated combinations of Flag-Smad2, Smad4-HA, Tax, and TβRI-T204D-HA. The top panel shows the receptor-dependent formation of heteromers containing Smad2 and Smad4, and the lower 2 panels show the expression of each protein as indicated.