Merlin (or schwannomin) is a tumor suppressor encoded by the neurofibromatosis type 2 gene. Many studies have suggested that merlin is involved in the regulation of cell growth and proliferation through interactions with various cellular proteins. To better understand the function of merlin, we tried to identify the proteins that bind to merlin using the yeast two-hybrid screening. Characterization of the positive clones revealed a protein of 749 amino acids named merlin-associated protein (MAP), which showed wide tissue distribution in Northern blot analysis. Sequence analysis revealed that MAP is a potential homologue of a yeast check-point protein, BUB2, and contains TBC, SH3, and RUN domains, thereby implicating its role in the Ras-like GTPase signal pathways. MAP and merlin were directly associated in vitro and in vivo, and colocalized in NIH3T3 cells. The RUN domain of MAP and the C-terminus of merlin appeared to be responsible for their interaction. MAP decreased the AP-1-dependent promoter activity additively with merlin in NIH3T3 cells. In addition, merlin and MAP synergistically reduced the colony formation of NIH3T3 cells. These results suggest that MAP may play a cooperative role in the merlin-mediated growth suppression of cells.