MUTATIONS INCREASING AUTOINHIBITION INACTIVATE TUMOR SUPPRESSORS SMAD2 AND SMAD4

Smad2 and Smad4 are related tumour-suppressor proteins(1,2), which, wh en stimulated by the growth factor TGF-beta, form a complex to inhibit growth(3). The effector function of Smad2 and Smad4 is located in the conserved carboxy-terminal domain (C domain) of these proteins and is inhibited by the presence of their amino-terminal domains (N domain)( 4,5). This inhibitory function of the N domain is shown here to involv e an interaction with the C domain that prevents the association of Sm ad2 with Smad4. This inhibitory function is increased in tumour-derive d forms of Smad2 and 4 that carry a missense mutation in a conserved N domain arginine residue. The mutant N domains have an increased affin ity for their respective C domains, inhibit the Smad2-Smad4 interactio n, and prevent TGF beta-induced Smad2-Smad4 association and signalling . Whereas mutations in the C domain disrupt the effector function of t he Smad proteins, N-domain arginine mutations inhibit SMAD signalling through a g-ain of autoinhibitory function. Gain of autoinhibitory fun ction isa new mechanism for inactivating tumour suppressors.