Tuberin phosphorylation regulates its interaction with hamartin - Two proteins involved in tuberous sclerosis

Hamartin and tuberin are products of the tumor suppressor genes, TSC1 and T SC2, respectively. When mutated, a characteristic spectrum of tumor-like gr owths develop resulting in the syndrome of tuberous sclerosis complex. The phenotypes associated with TSC1 and TSC2 mutations are largely indistinguis hable suggesting a common biochemical pathway. Indeed, hamartin and tuberin have been shown to interact stably in vitro and in vivo. Factors that regu late their interaction are likely critical to the understanding of disease pathogenesis, In this study, we showed that tuberin is phosphorylated at se rine and tyrosine residues in response to serum and other factors, and it u ndergoes serial phosphorylation that can be detected by differences in elec trophoretic mobilities. A disease-related TSC2 mutation (Y1571H) nearly abo lished tuberin phosphorylation when stimulated with pervanadate, Expression of this mutant tuberin caused a marked reduction in TSC1-TSC2 interaction compared with wild-type protein and significantly curtailed the growth inhi bitory effects of tuberin when overexpressed in COS1 cells, consistent with a loss of function mutation. Examination of a second pathologic mutation, P1675L, revealed a similar relationship between limited phosphorylation and reduced interaction with hamartin, Our data show for the first time that 1 ) tuberin is phosphorylated at tyrosine and serine residues, 2) TSC1-TSC2 i nteraction is regulated by tuberin phosphorylation, and 3) defective phosph orylation of tuberin is associated with loss of its tumor suppressor activi ty. These findings suggest that phosphorylation may be a key regulatory mec hanism controlling TSC1-TSC2 function.