Mice null for menin, the product of the multiple endocrine neoplasia type 1 (MEN1) gene, exhibit cranial and facial hypoplasia suggesting a role for menin in bone formation. We have shown previously that menin is required for the commitment of multipotential mesenchymal stem cells into the osteoblast lineage in part by interacting with the bone morphogenetic protein (BMP)-2 signaling molecules Smad1/5, and the key osteoblast transcriptional regulator, Runx2 (Sowa H., Kaji, H., Hendy, G. N., Canaff, L., Komori, T., Sugimoto, T., and Chihara, K. (2004) J. Biol. Chem. 279, 40267-40275). However, menin inhibits the later differentiation of committed osteoblasts. The activator protein-1 (AP-1) transcription factor, JunD, is expressed in osteoblasts and has been shown to interact with menin in other cell types. Here, we examined the consequences of menin-JunD interaction on osteoblast differentiation in mouse osteoblastic MC3T3-E1 cells. JunD expression, assessed by immunoblot, gradually increased during osteoblast differentiation. Stable expression of JunD enhanced expression of the differentiation markers, Runx2, type 1 collagen (COL1), and osteocalcin (OCN) and alkaline phosphatase (ALP) activity and mineralization. Hence, JunD promotes osteoblast differentiation. In MC3T3-E1 cells in which menin expression was reduced by stable menin antisense DNA transfection, JunD levels were increased. When JunD and menin were co-transfected in MC3T3-E1 cells, they co-immunoprecipitated. JunD overexpression increased the transcriptional activity of an AP-1 luciferase reporter construct, and this activity was reduced by co-transfection of menin. Therefore, JunD and menin interact both physically and functionally in osteoblasts. Furthermore, menin overexpression inhibited the ALP activity induced by JunD. In conclusion, the data suggest that menin suppresses osteoblast maturation, in part, by inhibiting the differentiation actions of JunD.