Mitogen-activated protein kinases (MAPKs) regulate a wide variety of cellular functions by phosphorylating their specific substrates. Here we have identified Tob as a novel substrate of MAPK. Tob, a member of the Tob and B-cell translocation gene anti-proliferative protein family, is shown to negatively regulate the proliferation of osteoblasts and T cells. In this study, our two-hybrid screening has identified Tob as an ERK2-interacting protein. Biochemical analyses have then shown that ERK MAPK (ERK2) and JNK/SAPK (JNK2) bind to and phosphorylate Tob in vitro. ERK catalyzes the phosphorylation more efficiently than JNK. When the ERK pathway is activated in cells, phosphorylation of Tob is induced. An ERK-binding or -docking site locates in the N-terminal portion of Tob, and phosphorylation sites reside in the C-terminal stretch region. The docking is crucial for efficient phosphorylation. Mutant forms of Tob, in which serines are replaced by glutamic acids to mimic phosphorylation, show a much reduced ability to inhibit the cell cycle progression to S phase from G(0)/G(1) phase, as compared with wild-type Tob, indicating that ERK phosphorylation negatively regulates the anti-proliferative function of Tob.