Moesin-ezrin-radixin-like protein (merlin) has long been considered a unique tumour suppressor that inhibits mitogenic signalling only at the membrane-cytoskeleton interface. However, the nucleocytoplasmic shuttling of merlin in a cell cycle-dependent manner has recently been observed, indicating that merlin may also exert its tumour-suppressive activity by interacting with specific nuclear protein partners. We have identified protein interacting with carboxyl terminus 1 (PICT-1) as a novel merlin-binding partner. Although the detailed mechanisms are not fully understood, several lines of evidence have previously implicated PICT-1 as a candidate tumour suppressor, including its phosphatase and tensin homolog deleted on chromosome 10 (PTEN)-dependent growth-suppression and cell-killing activities. We show here that PICT-1 is localised to the nucleolus, and Ser518-dephosphorylated merlin (the growth-inhibitory form of merlin) can interact with PICT-1 in the nucleolus. Ectopic expression of PICT-1, both in PTEN-positive HeLa cells and in PTEN-deficient U251 cells, effectively represses cyclin D1 expression, arrests the cell cycle at G0/G1, and promotes cell apoptosis. PICT-1 (1-356), a carboxyl-terminus truncated mutant that has lost the ability to bind merlin, has a markedly reduced inhibitory effect on the cell cycle and proliferation. Knockdown of merlin expression by siRNA attenuates the inhibitory effects induced by PICT-1 over-expression. We propose that merlin mediates PICT-1-induced growth inhibition by translocating to the nucleolus and binding PICT-1.
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