Selective interaction of plant homeodomain proteins mediates high DNA-binding affinity.

Understanding molecular mechanisms that control cell fate in the shoot apical meristem is a fundamental question in plant development. Genetic and molecular studies demonstrate that maize KNOTTED1 (KN1) of the TALE (3-aa acid loop extension) class of homeodomain (HD) proteins is involved in shoot apical meristem function. We show that KN1 interacts with knotted interacting protein (KIP), a BEL1-like TALE HD protein. Interaction between KN1 and KIP is mediated by conserved domains in the N termini of both proteins. The KN1 DNA-binding sequence, TGACAG(G/C)T, was biochemically identified, and in vitro DNA-binding assays show that individually KN1 and the HD of KIP bind specifically to this motif with low affinity. The KN1-KIP complex, however, binds specifically to this DNA-binding motif with high affinity, indicating that the association of KN1 and KIP may function in transcriptional regulation.